This is not expected behavior. You should be able to sign on a hosted file. Your problem could be related to a custom menu, access privileges, script, or a trigger.
It apparently had to do with the Available Menu Commands setting in the access privileges. It was set to minimal. Once I changed it to editing only, it worked. Interesting that users were able to enter data in other fields with the original setting, but not the container field.
Thanks for your quick response!
When you set the menu to minimal, FMGO can not access the insert options. If you want to keep the menus to minimal you can run a script set to run with full access priveges to do the insert.