After some reports of customers, I found a strange behavior.
These customers receive an internal server error page while accessing their account. I found that these errors in the server's errror_log:
End of script output before headers: index.php, referer: https://[my-domain]/customer/account/login/
To solve the problem with the account it is necessary to open the account in question in the backend and save it without making any changes.
After this procedure it is possible to log in from the frontend without any problem, at least for a while.
Has anyone else encountered this problem?
A similar behaviour was asked in this post, but the errors are a bit different, so i made another post. 1.9.3 Internal server error on account login