Old site, new error - but only for Chrome!

I've got a site that's been running on the same shared server for years and years.

I made a benign change to a single file today and the entire site has crashed - but only if you're using chrome. Firefox and Edge have no access problems.

I've cleaned out my cookies etc and used a couple of different chrome profiles - one of which had never accessed the site before - and all have the problem.

Error Logs:

2020-09-23 12:32:09 Error: [FatalErrorException] Allowed memory size of...

Code (markup):

Old site, new error - but only for Chrome!

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