The custom form part is a deprecated part since version 6.45.  We no longer support it due to the addition of enhancements in Common Forms.  Recreate this form as a Common Form.

To access the testconfig page:
If NetCommunity is on version 7.0 and below, go to: [netcommunityURL]/testconfig.aspx. 
If NetCommunity is on version 7.1 and above, refer to Access is denied warning when trying to view testconfig.aspx