Hello,
in the personal accounts settings there is the field "Non-numerical answer option prefix".

I tried to set it to another value and got the message that the format does not fit or need to be right. But I don't know what the format is because the error message does not tell it.

Expected behavior:
Make the expected format clear.
Communicate the format before an error appears.

Sidenote: On my system the prefix is by default set to "AO" (two letters). I am not sure if this is default to LimeSurvey or just to my instance. If it is upstream default please take it as an extra bug report. The prefix is followed by numbers; e.g. "AO01" (letter A, letter O, number 0, number 1). Having letter O and number 0 side by side makes it hard to read.