This will be difficult to solve, errors or permission problems with custom forms are very difficult to find.
You can try to unlink the user define fields, and start linking only one, then test the store all, then the second one, test store all etc...
This way you can hopefully detect the field causing the problem.
If it is not the custom field, it can be for example a custom-form, prohibiting creation of user-defined fields (GeniusConnect uses special user-defined fields to store primary key info in the Outlook item).
Microsoft Outlook is firing an exceptions/error without (any!) error description, this makes it difficult to detect the problem