When a instructor tries to add an assignment restricted to groups the lessons tool gets blocked.
It just happens if the database that is used is old and updated by upgrade scripts. There is a field that has to be clob that is a varchar. There wasn't a conversion script to make that conversion when the change came to the code.
It must be added to conversion scripts:
ALTER TABLE lesson_builder_groups ADD (tmpgroups CLOB);
UPDATE lesson_builder_groups SET tmpgroups=groups;
ALTER TABLE lesson_builder_groups DROP COLUMN groups;
ALTER TABLE lesson_builder_groups RENAME COLUMN tmpgroups TO groups;