I don’t recall the exact details, but some sort of custom code was the issue. If I remember correctly, they wanted to make chages to the query, which then ended up looping infinitely. Once the code was removed, everything went butter smooth.
This issue looks very imilar, but the exact cause is the question. Everything seems to work all right, until one point the process just hangs, and there are no more details in the front-end error console.
If you can add temporary FTP and back-end details, I can probably get to the bottom of it very quickly via code inspection.