.mda is an add-in file, no?
.mde is the "compiled" version of .mdb; accde the compiled version of accdb. It is odd that you say the file was developed in version 2016 yet is .mde
Application-defined or object-defined error, 2473
Such errors out of the blue are often caused by
a) badly written code that breaks after an update or
b) an update introduces a bug. Seems to be a common thing lately. If you just updated, consider trying to roll back and see if the issue goes away.
Error 6 may be due to your SO number being defined as integer in table design and you hit the limit (32767).
Aside from what I suggested re rollback, I think you'd need to fix the mdb file and remake an mda out of that. Backup these files ASAP and don't alter their design - alter the copies.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.