Just an eM Client user here.
My thought is that perhaps, if nothing was changed in eM Client, a software program, email server issue or computer hardware might be the problem. Any of these can make correcting the issue difficult.
Software could be a type of anti-virus/malware program, or just another piece of newly installed software that doesn’t work well with others. I had this problem with my Samsung SSD Magician software whenever I used it - it would prevent me from actually playing an online game I play daily (retired). Once I figured out it was the problem, I stopped using it, as it was only a utility to check the health of my SSD. Strange occurrence but sometimes other software can be a problem, though rare.
With an email server issue, you should check with them about settings and if there are some sort of limits with attachments, but assuming they are not the problem, something on your computer is doing the blocking, and it may not be eM Client.
A computer hardware issue would be an example of a failing hard drive. When a mechanical hard drive starts to go bad all sorts of problems can start happening that appear to be software related. Software programs can get corrupted with bad writes to disk, for example. A failing hard drive usually doesn’t happen until about 3 years old or older, but can happen anytime. There are free utilities out there on the Internet to check your hard drive, if you would like to eliminate that. I, personally, use Hard Drive Sentinel, which can be downloaded free to use to do a simple check of the drive.
However, your first step might be, especially since eM Client seems corrupted in some way, to uninstall, do a reboot and reinstall. If nothing changes with the crashing on doing blacklists then something on your computer is causing the problem. If, when you did a reinstall, you can choose the folder eM Client creates to install itself in, do another reinstall and change the name or location for it - that will allow it to actually be installed in a totally different location on your hard drive, and should fix any data corruption within eM Client.
Just some ideas that might, or might not help… Hope you find the fix.