Hi @kimsvane and @Holla_Die_Waldfee,
Indeed this is similar to Why are session timing out more rapidely since v2.11? . So this behavior is happening because since 2.11 we are actually more strict with session management (no auto-extension). But we understand that it introduces a confusion from a user standpoint between session duration and passphrase remembering.
We are working on an improvement which will be shipped most likely with the next release. With this fix, the session will not expire for as long as the passphrase is remembered. If "remember until I log out’ is selected, then the session will be kept active until the system gets in idle mode, the browser is closed or a manual logout is done.
In the meantime, extending the session time from the configuration should fix the issue.