How to reproduce

1. Apply the attached resource pack 'Crash.zip'
2. Create a new world
3. Run command: 

   /setblock ~ ~ ~ minecraft:sign{Text1:"{\"text\":\"M\"}"}

4. Your game should crash

Why does it happend (my idea)

Text in Minecraft signs is limited by character width (max. 15 'M' characters in one line)
Even if you try

/setblock ~ ~ ~ minecraft:sign{Text1:"{\"text\":\"MMMMMMMMMMMMMMMMMMMMM\"}"}

There will be only 15 M characters on the sign.
These extra characters are still stored in the sing (can be seen with /data get block <coordinates>).

My resource pack 'Crash.zip' makes the M character 100 px long.
If you try to write this character in a sign, it doesn't work, nothing happens.

But when you "force" it into a sign with /setblock , the game tries to hide characters that don't fit into a sign, fails doing this and crashes.