It seems we are experiencing this bug in macOS.
Quote from Apple:
I confirmed our engineering team is aware of this issue and I expect it will be addressed in an upcoming software update. I will follow up once we have more information.
Meanwhile we are doing our best to come up with some workaround for that. Sorry for all the inconvenience, but it seems it is currently out of our control, but we’ll do best to resolve it somehow anyways.
Thank you, @Michal_Burger. It’s good to know the cause may come from the OS rather than eM Client (which would explain why beta testing might not have picked up on this previously) and that Apple is very aware. Hopefully they’ll release 15.4.1 soon!
I also appreciate you’re doing what you can quickly, the apologies, and the communication about what is going on. Sometimes simply “not being in the dark” can already be a big help for customers! 
The link to the reported issue from Eclipse users is here:
A temporary workaround might be to disable conversations until there is a better workaround/solution.
10.3.1524
I wanted to provide an update on two matters.
To make everyone aware, Michal has mentioned elsewhere that version 10.3.1567 is now available, with a few bug fixes (although not anticipated to overcome the macOS 15.4 bugs). From messages above, it sounds like we’re awaiting Apple to address an issue with a macOS update. Hopefully they will do soon.
Release History | eM Client
I’ll add an observation, in case it is helpful. I’ve been running eM Client v10.1.4828 on the desktop which is now running macOS 15.4 (enforced update at work this work, after running 15.3 as mentioned higher up this thread). Yesterday, I had lighter than normal usage of eM Client but encountered no crashes at all. This surprised me, as I might have expected the macOS 15.4 issues to affect 10.1 as well. I’m running it again today and will let you know if I encounter issues.
Ok, please let us posted. We were able to reproduce the macOS 15.4 issue on both 10.1 and 10.3. Of course I cannot guarantee for 100% that there is no 10.3 only issue as well.
And yes we are waiting for a fix from Apple but meanwhile (as a highest priority) we are further diagnosing the issue, if there is a workaround that would work even with the unpatched 15.4 macOS.
Sure, will do! 
The build of macOS 15.4 is 24E248, and I presume that is the same for everyone. I started out a little carefully, as I didn’t want to induce a crash, but so far no issues. I’ve also tried double-clicking on a message to open it in a new window, which had previously crashed the app, but that was fine.
Is there any particular information that would be useful to the team? I’m happy to provide whatever might be of help.
Weirdly, this is the second day of running 10.1.4828 under macOS 15.4 and there have been no crashes.
I encountered one very strange error which was: “[Exchange Web Services] The operation can’t be performed because the item is out of date. Reload the item and try again.” But I presume that was unrelated (server issue?) and it didn’t reoccur anyway.
Is there anything the team have found is guaranteed to cause a crash with v10.3? If so, I can try that for you and see if it happens here with v10.1 on macOS 15.4. I’m happy to help with the troubleshooting as best I can.
It’s probably best to avoid this update: Update today broke EmClient - #8 by Crazy4Happy
Maybe for now, but 10.3 does bring new features and I trust there will be a fix for the situation soon. I’ll stick with v10.1 on a desktop until the fix is clear, and am testing v10.3 on my laptop(s); all are running macOS 15.4.
I’ve just received a software update notification: macOS 15.4.1 is out. It seems to be a security update, though.
The problem is still not resolved in 15.4.1
Ah, that’s a shame, but I sort of expected that. Thanks for the very quick update!
thanks for the update
Posting updates to keep the eM Client team informed during troubleshooting:
I used 10.1.4828 for three days under macOS 15.4 on my desktop and I didn’t encounter any crashes or other issues. It was lighter than normal usage, but still enough to test. I was expecting the macOS 15.4 issue would bring down eM Client 10.1 too but haven’t experienced it yet.
I used 10.3.1567 under macOS 15.4 on a laptop, testing it very lightly for a day or two. It didn’t crash on its own and I was able to click between different views (mail, calendars, attachments, contacts, etc.) without problems. It got to the point where I wondered if the issues might have disappeared, strangely! I tried double-clicking on a message to open it in a new window and it worked. I tried maybe three or four times without problems, and was surprised. I tried again and then the app crashed.
I’m anticipating we’re simply waiting for an update from Apple, but I’ll post experiences here in case anything might be useful for further troubleshooting in parallel.
Thank you for your report. Based on our data it should behave similarly on 10.1, so it would be good if you can try the same procedure that led to crash in 10.3 on 10.1 (we are able to reproduce it on 10.1 too, maybe it is just less common).
We are still trying to find a workaround for this, but it is very hard to do so.
I very much appreciate all your efforts and those of the eM Client team. Thank you. 
Sure, I will test v10.1. I’m travelling and will next have a short chance to test it tomorrow. I’ll try opening a message in a new window because that seems to give me the highest chance of a crash so far (otherwise it seems random). Are there any specific actions you’d like me to try, to reproduce what you’re seeing?
I’m happy to do all I can to help!
The crashes are now occurring every hour and sometimes more frequently. I am going to follow the threads but since it has become so problematic I am going to have to stop using em Client. This makes me incredibly sad since I thought I finally had a client that worked for both gmail (personal mail) and work