Heureka, it seems we have a breakthrough. We’ve found a workaround for that issue and it is included in this version:
https://www.emclient.com/dist/v10.3.1720_Mac/setup.pkg
Can you please try it and confirm it really resolves the issue?
It was super hard to find it, but by disabling an undocumented cache in webkit the macOS 15.4 bug/crash seems to be bypassed. It works for us, hopefully it will work for you too.