@Traumflug: I did not play around with it for too long, as I aborted the testing when (luckily) a new customer contacted us through FB telling us he could not create an account / log into our main store.
I can provide you with the info I got though, if you can make something out of it.
We installed a new shop in a subfolder a while back with a new database for this store, as we're able to have several databases on our hosting plan. Seemed to work fine, until we were settting up payment solutions (paypal and stripe). To give these a test, we tried making an account, but nothing happened on the login pages. No Error, no nothing. Just didn't take you to the "your page"-site. This happened on Chrome and Firefox. (If I'm not mistaken I believe my partner managed to log in using Vivaldi and Edge....)
Thinking it might be a SSL-issue with security within the browsers, I thought I could try turning on SSL in BO, which seemed to be a solution, as we both now managed to log in using Chrome and FF.
Believing this issue now was solved we called it a day. Early next moring this customer had messaged us through our FB- page telling us he could not create/log in in our main store. Knowing that this most likely was related to the subdomain store, I logegd in and turned off the SSL in the subdomain store, and told him to try again, and now it worked,
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