The application misbehaviour is a complex interaction ...
Maybe it looks like that but is something very simple ... Like app doing GetAttr() with uninitizalized variable and object/gadget does not handle/know attribute in OM_GET. Maybe try adding some temp debug output warning to Intuition/GetAttr() (if taskname is "checkitout") (make it call the OM_GET method twice, once on a temp storage with preset magic value, once on the real storage pointer).
It is known that boopsi based gadtools.library causes problems with stupid apps which rely on standard AOS non-boopsi based gadtools.library internal implementation, like checking for GTYP_STRGADGET (reqtools.library). And maybe other things like order of sub gadgets.