https://xoops.org/modules/newbb/viewtopic.php?topic_id=11136&forum=7
Since it 'suddenly started happening', it really sounds like a database error as suggested in the above thread. Have you run 'check table <table_name>' and/or 'repair table <table_name>' for the relevant tables? You will need some way to access the database such as command-line MySQL or phpmyadmin or mysqlfront or something like that...