I'll try bumping the memory. Although the "months of not working" is not quite correct, because it worked fine with 10 GB in 5.6.
After a strange crash yesterday that didn't even print an error message [0,1] it seems we're back to normal now.
I still don't quite understand why the memory needs to be increased so much. I thought that that's not necessarily a good thing because it means heap compactification occurs less often and hence might increase GC runtime etc.
[0]: https://ci.isabelle.systems/jenkins/job/isabelle-nightly-slow/586/consoleFull [1]: https://ci.isabelle.systems/jenkins/job/isabelle-nightly-slow/586/artifact/heaps/polyml-5.7.1_x86_64-linux/log/Iptables_Semantics_Examples_Big/*view*/