XF 2.3 Problems using ACP - crashing

[Alvin63]

Since yesterday, if I try to enter ACP, save anything in ACP or even approve something in the manual approval queue, the site "crashes" with the warning attached below. If a I wait a minute or so and click "try again" I get in /it gets saved. But it happens repeatedly. I did edit some html in a couple of places within ACP yesterday but I think it started before that. However I assumed it might have been editing the html had caused the server to tighten security, so contacted the server to ask if there were any restrictions since yesterday - they said can't see any issues their end. But it's still happening. Any ideas? If I follow the "learn more" link from the screenshot below (the screenshot is the error page that comes up), it says something about SSLKEYLOGFILE. Link to that page below where it describes it.

Thanks.

Client Challenge

 

[Alvin63]

I actually think it might have started before I edited any html so it might just be coincidence - it just happened when I logged into ACP. After discussion with server and asking them to look into if a specific rule in mod security was causing a false positive. They got back and said nothing related to mod security rules - the forum is regularly hitting its CPU resource limit. And my site is seeing very heavy requests for a single file service_worker_js that is being fetched more than would normally be expected - they say it's a Xenforo configuration matter on the caching/service.

So any idea what that could be? One of my addons? Too many visitors?

They specifically said very heavy request volume from service_worker.js