Machine behaves like child lock has been activated

Hi all, we've got a five year old Siemens WM14T5EM, which suddenly started to behave like the child lock has been activated. After turning on the machine no input is possible and the machine will answer every input with two beeps. The child lock is not (!) activated. Trying to deactivate the child lock does not work, because that also results in the "beep".

The error seems to occur sporadically, because sometimes it does not occur. Then the machine behaves like normal. Most of the time though, after turning it off and on again, the error reappears.

Does anybody have an idea what that could be? Thanks in advance!