I played with it a little more, and it is obviously a bug in SSMS. I can't see that SSMS submits anything to SQL Server and waits for response. It submits a metadata query which returns successfully.
I tested this mainly with SSMS 19 Preview 3, in hope that the issue would not appear. And indeed, it did not at my first test, but the contents had been altered due to encoding issues. I also found that the issue may not appear if I perform some other thing first. But if I go straight to the megjegyzes column and enter one character at the end and press Enter, SSMS goes into nirvana.
You can report bugs related to SQL Server here:
https://feedback.azure.com/d365community/forum/04fe6ee0-3b25-ec11-b6e6-000d3a4f0da0
Unfortunately, I don't think they accept attachments, but you could link to this thread.
I would also say that I found it quite cumbersome to update that long column through the grid. A plain UPDATE statement is easier to me.