I hesitate to suggest the obvious for you certainly are not a novice. Since there's no query involved (as I interpret your post) I'll leave query out of the mix.
One of the new fields is a calculated field?
Table was corrupted and one or more indexes were removed (try c/r)
db is being opened read only?
Some IT guy messed with the folder permissions (playing with Active Directory?)
I would copy the db and test the copy. Then remove added fields and retest. Then put them back in one by one and retest as you go. You might find that one in particular is the culprit.
I also assumed this is an Access be and not Sql Server or the like.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.