I get the same error message for questions with many entries (8x6 matrix (48 entries)) AND with achievements enabled AND for incorrect answers. For smaller matrices or with achievements deleted or with correct answer there is no error message.
Debugging a (simple?) product rule question - answer submission not working right
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 5 posting(s) in 1 day