I have no idea why.
mod - 2.4.0
crash-2020-06-04_17.32.07-server.txt

I've tried reproducing this myself and it feels very on occasion for me (it happens when I break the double variant). Try out the latest version and see if the error still persists.

Client is still dying

Same error?

Yes
crash-2020-06-07_20.08.08-client.txt

Can you please check this still occurs in the latest version?

It works! It seems that somehow it does not break correctly with visual bugs, and sometimes it is absolutely normal, but it breaks and does not crash.
Sometimes after breaking a block a technical texture appears or the breaking animation freezes

Yeah I've noticed this myself but I'm not too sure how I can fix this. I'll leave this issue open cause I've only fixed the crash and not the visual issue

@CJMinecraft01 I'd rather create a new one. Another problem anyway.

Same Issue

Log: 2020-06-04-2.log.gz

Crash Report: crash-2020-06-04_14.15.10-server.txt

Should be fixed in the latest version! Sorry about that (I'd left some old redundant code in)

@CJMinecraft01 Sorry but new vertical slab feature still trigger server crash and massive lags. Maybe it was worth creating a new crash report, but the problem is almost the same.
Mod - 2.4.1
crash-2020-06-05_00.09.52-server.txt

I think I've spotted the issue and that should be resolved in the latest version! Sorry about that. Let me know if there are any further issues

Do you have the log to see what has errored? Also, which slabs were you trying to combine and which were you trying to break?

Ok, the client on server starts to lag when I try to break the vertical slab. More precisely, the client simply dies. Heals only alt + f4. Is it normal that vertical slabs cannot be doubled?
Edit: Hmm... For some reason this is not happening anymore. Something is wrong here.

Sorry I forgot crash report and logs attached. I used prismarine slab, but I think which block I used is not very important.
crash-2020-06-05_14.33.29-client.txt
2020-06-05-2.log

I repeated the error. In survival mode, I break with a pickax cobblestone slabs like last time. Vertical slabs connected one to the other and I tried to break one of them and got the same error. Another time, I did not connect them, but I got the same error.
crash-2020-06-05_21.47.17-client.txt
latest.log

Also, which slabs were you trying to combine and which were you trying to break?

My mistake is the second slab is put without pressing shift