This is becoming a comedy of errors.

We have been contacted by several plan participants who say that the company has been out of business for 2 years and no one can find the sole trustee. However, we have recently received and processed distributions for other participants. Wondering whether someone had actually found the trustee, or someone had forged the trustee's signature, we pulled the plan document to compare the signatures. What we found is that the trustee never signed the document when the plan was adopted.

Any suggestions how to fix this?

I would suggest contacting the DOL abandoned plan program (EBSA office) where the plan is located and providing them with the informaiton. Let the DOL contact the trustee.