oh la!
Güther, please don't feel sorry!
I was very eager to try to figure out the problem and learn something from it. It was good to learn that Safari and Firefox seem to have slightly different ways of handling Private Browsing and to learn how that setting interacts with their other settings.
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Earlier, the SMF software did have a bug involving the password hash. This bug would bump someone out of their login with an erroneous "Password Incorrect" message. But that was fixed long ago. So it is not clear what might be causing any similar problems currently.
I have absolutely no way to determine whether a Member is entering their password correctly or incorrectly. Our Error Logs only report Password Incorrect but do not report what was typed - for obvious security reasons benefiting members. So all we can do is to check a member's browser settings and try to reproduce the problem on our own machines under similar settings.
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Hans, I'm thinking that it might be useful to reset your current password and
temporarily choose a very, very easy-to-enter password with only letters and numbers and NO special characters. Then after logging on with it, if you get bumped off right away, you would be even more certain that it was not fumblefingers which caused the Password Incorrect message. [I myself have fumbled even simple abc123 passwored more than once!! We all do fumblefingers on occasion.]
Please understand this is just a testing suggestion and does not imply that you are constandly fumbling your current password, OK???
After a short test with the simple password, change it back to your more secure password.