I likewise ran into the borrowing bug for the first time last week, and I have a mix of resale and direct and multiple contracts across multiple home resorts. I'm going to send this to Disney to see if it helps with troubleshooting, but my experience was:
1. I modified an existing reservation (at BLT) within 7 month window, and borrowed points from 2 resale contracts (SSR). This went through fine, and the points were borrowed.
2. I went into the same reservation again to modify it AGAIN, and those 2 resale contracts showed that I could borrow the full 50% amount, despite the fact that I borrowed already from them. If I tried to borrow ANYTHING when modifying this reservation, I got the 7 dwarves. I tried a good 20 times; no go.
3. I tried to create a NEW reservation using one of the 2 resale contracts I had already borrowed with, keeping it under 50%. No luck. Always 7 dwarves. This was a reservation within 7 months, borrowing from a resale contract that was a different home resort (SSR) than the resort of the reservation (VBR)
4. I then went in to modify an existing reservation (CCV) within the 11 month priority window to borrow points from 3 different CCV resale contracts, and was able to do so without issue.
Now, I still had to call in, because I was shuffling points around and wanted to borrow as much as possible from my SSR contracts so I was borrowing as little as possible from CCV, but these are my data points.
It seems to me that something happened when borrowing from SSR where the contracts did not get marked correctly as having been borrowed against, and whatever that is, is preventing those contracts to be used for borrowing now. It also possible if trying to borrow points from a contract that doesn't match the resort of the reservation it errors, even if I'm within the 7 month window - perhaps there is some bug in the logic for checking for borrowing restrictions that they would have needed to implement for RIVIERA resale contracts, and it is affecting ANY resale that meets a certain criteria?