retrieve_ajax suddenly not working in my active project

I've been running a script for a good 1 year now. However the retrieve_ajax seems to have problems just today. It's basically a document filing system that uses multiple sheets to log data. On Friday the code was working properly and with no issue, but come 11AM (GMT + 8), the system seems to start bogging down, until it suddenly comes to a total stop, with the error message I put in for failure to retrieve the sheets popping up.

Any suggestions what can be done on this?