From the exception, it looks like there is some problem at the server end, but still I will ask you to do the below.
Can you please ensure the following?
1. Check whether the form is really existing or not. For this you can close the form list Object view. Clear the cache and then relaunch the object list. If the form is existing you will find it in the Object List view.
2. If it is existing, then you can try deleting the form using java driver and see whether you are able to delete the object.
3. Check whether the record exists for your form in arschema table and the corresponding T, B, and H Table also exists.
If there is no problem then you can attach your api, SQL logs for further analysis.
Just to complement the 3rd bullet, if you do not know how to consult in arschema, run this query:
select name, schemaid from arschema where name = 'FORM_NAME' confirm that there are TXXXX (table), BXXXX (blob), HXXXX (history)