Answer
This class of error typically comes from an operation attempting to combine or compare two genuinely incompatible data types somewhere in the executed logic. As a diagnostic starting point, try temporarily disabling any customizations associated with that specific screen and its Logical Unit — custom fields and custom events in particular — to check whether a customization (rather than standard IFS logic) is introducing the type mismatch. If the issue can't be reproduced for a standard/core account like IFSAPP but persists for the affected user, also try restoring that user's individual profile and base profile, since a corrupted or unusual profile state can sometimes trigger this kind of error for one specific user only.