Thank you very much for this hint. The normal error log had nothing to offer. Switched SSL engine log level debug on.
The result is not of help to me. Suppose the problem is here: AH00566: request failed: malformed request line. Even google has no answer for this one but all the results from google point to some sort of proxy problem. After all it seems to be a problem with the Oracle Cloud Server environment. For reference sake log-file included.
[Wed Apr 15 08:54:56.429622 2020] [ssl:info] [pid 1761] AH01876: mod_ssl/2.4.29 compiled against Server: Apache/2.4.29, Library: OpenSSL/1.1.1
[Wed Apr 15 08:54:56.484136 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(401): AH00821: shmcb_init allocated 131072 bytes of shared memory
[Wed Apr 15 08:54:56.484152 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(417): AH00822: for 130984 bytes (131072 including header), recommending 4 subcaches, 21 indexes each
[Wed Apr 15 08:54:56.484155 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(450): AH00824: shmcb_init_memory choices follow
[Wed Apr 15 08:54:56.484157 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(452): AH00825: subcache_num = 4
[Wed Apr 15 08:54:56.484160 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(454): AH00826: subcache_size = 32744
[Wed Apr 15 08:54:56.484162 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(456): AH00827: subcache_data_offset = 520
[Wed Apr 15 08:54:56.484164 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(458): AH00828: subcache_data_size = 32224
[Wed Apr 15 08:54:56.484166 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(460): AH00829: index_num = 21
[Wed Apr 15 08:54:56.484179 2020] [socache_shmcb:info] [pid 1772] AH00830: Shared memory socache initialised
[Wed Apr 15 08:54:56.484191 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(401): AH00821: shmcb_init allocated 512000 bytes of shared memory
[Wed Apr 15 08:54:56.484206 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(417): AH00822: for 511912 bytes (512000 including header), recommending 32 subcaches, 88 indexes each
[Wed Apr 15 08:54:56.484209 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(450): AH00824: shmcb_init_memory choices follow
[Wed Apr 15 08:54:56.484211 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(452): AH00825: subcache_num = 32
[Wed Apr 15 08:54:56.484213 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(454): AH00826: subcache_size = 15992
[Wed Apr 15 08:54:56.484215 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(456): AH00827: subcache_data_offset = 2128
[Wed Apr 15 08:54:56.484217 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(458): AH00828: subcache_data_size = 13864
[Wed Apr 15 08:54:56.484220 2020] [socache_shmcb:debug] [pid 1772] mod_socache_shmcb.c(460): AH00829: index_num = 88
[Wed Apr 15 08:54:56.484292 2020] [socache_shmcb:info] [pid 1772] AH00830: Shared memory socache initialised
[Wed Apr 15 08:54:56.484315 2020] [ssl:info] [pid 1772] AH01887: Init: Initializing (virtual) servers for SSL
[Wed Apr 15 08:54:56.484345 2020] [ssl:info] [pid 1772] AH01914: Configuring server server6.servo-tec.com:443 for SSL protocol
[Wed Apr 15 08:54:56.484350 2020] [ssl:debug] [pid 1772] ssl_engine_init.c(1705): AH: Init: (server6.servo-tec.com:443) mod_md support is unavailable.
[Wed Apr 15 08:54:56.484769 2020] [ssl:debug] [pid 1772] ssl_engine_init.c(1989): AH02209: CA certificate: CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US
[Wed Apr 15 08:54:56.484978 2020] [ssl:debug] [pid 1772] ssl_engine_init.c(1118): AH01904: Configuring server certificate chain (1 CA certificate)
[Wed Apr 15 08:54:56.484985 2020] [ssl:debug] [pid 1772] ssl_engine_init.c(492): AH01893: Configuring TLS extension handling
[Wed Apr 15 08:54:56.485199 2020] [ssl:debug] [pid 1772] ssl_util_ssl.c(470): AH02412: [server6.servo-tec.com:443] Cert matches for name 'server6.servo-tec.com' [subject: CN=server6.servo-tec.com / issuer: CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US / serial: 04BBD791C75F48379F495B5E4DC6C68DFA5A / notbefore: Apr 13 05:31:45 2020 GMT / notafter: Jul 12 05:31:45 2020 GMT]
[Wed Apr 15 08:54:56.485226 2020] [ssl:info] [pid 1772] AH02568: Certificate and private key server6.servo-tec.com:443:0 configured from /etc/ssl/froxlor-custom/server6.servo-tec.com.crt and /etc/ssl/froxlor-custom/server6.servo-tec.com.key
[Wed Apr 15 08:54:56.485321 2020] [ssl:info] [pid 1772] AH01876: mod_ssl/2.4.29 compiled against Server: Apache/2.4.29, Library: OpenSSL/1.1.1
[Wed Apr 15 08:54:56.485347 2020] [watchdog:debug] [pid 1772] mod_watchdog.c(454): AH010033: Watchdog: Running with WatchdogInterval 1000ms
[Wed Apr 15 08:54:56.485352 2020] [watchdog:debug] [pid 1772] mod_watchdog.c(462): AH02974: Watchdog: found parent providers.
[Wed Apr 15 08:54:56.485356 2020] [watchdog:debug] [pid 1772] mod_watchdog.c(508): AH02977: Watchdog: found child providers.
[Wed Apr 15 08:54:56.485358 2020] [watchdog:debug] [pid 1772] mod_watchdog.c(516): AH02978: Watchdog: Looking for child (_singleton_).
[Wed Apr 15 08:54:56.485362 2020] [watchdog:debug] [pid 1772] mod_watchdog.c(516): AH02978: Watchdog: Looking for child (_default_).
[Wed Apr 15 08:54:56.488694 2020] [mpm_prefork:notice] [pid 1772] AH00163: Apache/2.4.29 (Ubuntu) OpenSSL/1.1.1 configured -- resuming normal operations
[Wed Apr 15 08:54:56.488710 2020] [mpm_prefork:info] [pid 1772] AH00164: Server built: 2020-03-13T12:26:16
[Wed Apr 15 08:54:56.488717 2020] [core:notice] [pid 1772] AH00094: Command line: '/usr/sbin/apache2'
[Wed Apr 15 08:54:56.488720 2020] [core:debug] [pid 1772] log.c(1570): AH02639: Using SO_REUSEPORT: yes (1)
[Wed Apr 15 08:54:56.488723 2020] [mpm_prefork:debug] [pid 1772] prefork.c(919): AH00165: Accept mutex: sysvsem (default: sysvsem)
[Wed Apr 15 08:54:56.525530 2020] [watchdog:debug] [pid 1778] mod_watchdog.c(565): AH02980: Watchdog: nothing configured?
[Wed Apr 15 08:54:56.525641 2020] [watchdog:debug] [pid 1777] mod_watchdog.c(565): AH02980: Watchdog: nothing configured?
[Wed Apr 15 08:54:56.526269 2020] [watchdog:debug] [pid 1775] mod_watchdog.c(565): AH02980: Watchdog: nothing configured?
[Wed Apr 15 08:54:56.526771 2020] [watchdog:debug] [pid 1776] mod_watchdog.c(565): AH02980: Watchdog: nothing configured?
[Wed Apr 15 08:54:56.527876 2020] [watchdog:debug] [pid 1774] mod_watchdog.c(565): AH02980: Watchdog: nothing configured?
[Wed Apr 15 08:59:29.772117 2020] [watchdog:debug] [pid 1825] mod_watchdog.c(565): AH02980: Watchdog: nothing configured?
[Wed Apr 15 09:04:39.906478 2020] [core:debug] [pid 1777] protocol.c(1278): [client 162.xxx.128.xxx:49410] AH00566: request failed: malformed request line
[Wed Apr 15 09:10:39.891356 2020] [core:debug] [pid 1774] protocol.c(1278): [client 107.xxx.23.xxx:47495] AH00566: request failed: malformed request line
[Wed Apr 15 09:12:32.955657 2020] [core:debug] [pid 1825] protocol.c(1278): [client 45.xxx.108.xx:534] AH00566: request failed: malformed request line
[Wed Apr 15 09:15:39.793681 2020] [core:debug] [pid 1778] protocol.c(1278): [client 130.xxx.xxx.xxx:48720] AH00566: request failed: malformed request line
[Wed Apr 15 09:15:45.464095 2020] [core:debug] [pid 1777] protocol.c(1278): [client 130.xxx.xxx.xxx:48742] AH00566: request failed: malformed request line
[Wed Apr 15 09:15:47.819096 2020] [core:debug] [pid 1775] protocol.c(1278): [client 130.xxx.xxx.xxx:48744] AH00566: request failed: malformed request line
[Wed Apr 15 09:15:49.463358 2020] [core:debug] [pid 1776] protocol.c(1278): [client 130.xxx.xxx.xxx:48746] AH00566: request failed: malformed request line