So I tried logging on and off several times, this didn't help. Having logged in I can open another tab and don't need to log in again, so this is working.
Then I checked the cookies my browser saved from pool.biblepay.org and I think I found the mistake: the credentials_password cookie saves a wrong value for my pw (double-checked this by deleting it, logging off and on again, same mistake). The problem seems to be that my pool-pw uses a semicolon ( ; ) in the middle of the word and this seems to create a problem for the cookie, because the cookie-pw stops at the letter before the ";". I will try and change my pw again without diacritical signs and see if that helps.
One of my computers just crashed again, I disabled SSL, let's hope for the best. :-D
Edit:
Oh, you beat me to it by posting the update, I'm gonna give this a try then, let's see how it works out over night.

Seems to have worked as expected over night. I tried 3 different things:
a) one machine with 1.0.7.5 and SSL enabled --> crashed
b) one machine with 1.0.7.5 and SSL disabled --> worked
c) one machine with 1.0.7.6 and SSL enabled --> worked
So this was definitely and solely an SSL issue, which is fixed in 76; thanks again.
Regarding the pool login: now without the semicolon in my password the pool page works like a charm and does not kick me out every couple of minutes, so problem solved I guess.

Only "problem" remaining is: why is the (only existing) pool hitting less than 20% of the blocks.

So I updated to 1.0.7.6 and most miners keeped working.
However some crashed.
I don't see a direct reason why... there is no error or exception.
Any idea?
2018-01-04 23:02:32 CMasternodePayments::FillBlockPayee -- Masternode payment 693702923735.000000 to BNidV9kWAZECM1sEtg7ZiQyYn2ghDY6mN9
2018-01-04 23:02:32 CMasternodePayments::FillBlockPayee -- Masternode payment 693702923735.000000 to BNidV9kWAZECM1sEtg7ZiQyYn2ghDY6mN9
2018-01-04 23:03:47 CMasternodePayments::FillBlockPayee -- Masternode payment 693702926015.000000 to BNidV9kWAZECM1sEtg7ZiQyYn2ghDY6mN9
2018-01-04 23:04:07 CMasternodePayments::FillBlockPayee -- Masternode payment 693702926015.000000 to BNidV9kWAZECM1sEtg7ZiQyYn2ghDY6mN9
2018-01-04 23:04:26 CMasternodePayments::FillBlockPayee -- Masternode payment 693702926015.000000 to BNidV9kWAZECM1sEtg7ZiQyYn2ghDY6mN9
2018-01-04 23:04:31 CMasternodePayments::FillBlockPayee -- Masternode payment 693702926015.000000 to BNidV9kWAZECM1sEtg7ZiQyYn2ghDY6mN9
2018-01-04 23:07:38 msghand thread interrupt
2018-01-04 23:07:38 addcon thread interrupt
2018-01-04 23:07:38 opencon thread interrupt
2018-01-04 23:07:38 net thread interrupt
2018-01-04 23:07:38 scheduler thread interrupt
2018-01-04 23:07:38 mnbcon thread interrupt
2018-01-04 23:07:39 PrepareShutdown: In progress...
2018-01-04 23:07:39 StopNode()
2018-01-04 23:07:51 Verifying mncache.dat format...
2018-01-04 23:07:51 Loaded info from mncache.dat 93ms
2018-01-04 23:07:51 Masternodes: 84, peers who asked us for Masternode list: 0, peers we asked for Masternode list: 0, entries in Masternode list we asked for: 0, masternode index size: 84, nDsqCount: 15347
2018-01-04 23:07:51 Writting info to mncache.dat...
2018-01-04 23:07:51 Written info to mncache.dat 690ms
2018-01-04 23:07:51 Masternodes: 84, peers who asked us for Masternode list: 0, peers we asked for Masternode list: 7, entries in Masternode list we asked for: 0, masternode index size: 84, nDsqCount: 15472
2018-01-04 23:07:51 mncache.dat dump finished 784ms
2018-01-04 23:07:51 Verifying mnpayments.dat format...
2018-01-04 23:07:52 Loaded info from mnpayments.dat 484ms
2018-01-04 23:07:52 Votes: 24388, Blocks: 2541
2018-01-04 23:07:52 Writting info to mnpayments.dat...
2018-01-04 23:07:54 Written info to mnpayments.dat 1807ms
2018-01-04 23:07:54 Votes: 25812, Blocks: 2684
2018-01-04 23:07:54 mnpayments.dat dump finished 2293ms
2018-01-04 23:07:54 Verifying governance.dat format...
2018-01-04 23:07:55 Loaded info from governance.dat 1295ms
2018-01-04 23:07:55 Governance Objects: 15 (Proposals: 13, Triggers: 1, Watchdogs: 1/1, Other: 0; Seen: 69), Votes: 0
2018-01-04 23:07:55 Writting info to governance.dat...
2018-01-04 23:07:56 Written info to governance.dat 773ms
2018-01-04 23:07:56 Governance Objects: 15 (Proposals: 13, Triggers: 1, Watchdogs: 1/1, Other: 0; Seen: 70), Votes: 691
2018-01-04 23:07:56 governance.dat dump finished 2068ms
2018-01-04 23:07:56 Verifying netfulfilled.dat format...
2018-01-04 23:07:56 Loaded info from netfulfilled.dat 40ms
2018-01-04 23:07:56 Nodes with fulfilled requests: 8
2018-01-04 23:07:56 Writting info to netfulfilled.dat...
2018-01-04 23:07:56 Written info to netfulfilled.dat 0ms
2018-01-04 23:07:56 Nodes with fulfilled requests: 9
2018-01-04 23:07:56 netfulfilled.dat dump finished 40ms
2018-01-04 23:08:17 Shutdown: done
Its possible that 1075 let a bad block in (not a security problem, but marked as bad) and that prevents the tip from being read, possibly, please try deleting chainstate -R and blocks -R on that particular node.