QuantumNoisemaker

For those curious about the specific commands; I found a better command for this that allows you to directly copy a double over to the command store, which doesn't appear to have the above issue:

/data modify storage horsestats:scan horse_speed set from entity @e[type=horse,limit=1,distance=..48,sort=nearest] Attributes[{Name:"minecraft:generic.movement_speed"}].Base

The above issue still stands though, if you need to convert a double over to an integer.

Thanks for the fix!

This seems to be a similar issue to the one described in MCL-17220

Can confirm, getting this exact issue: 

The new launcher update bricked my fabric loader launcher settings. I run Minecraft fabric with Java 15 instead of the pre-installed Java 8 JVM. This was all working fine until my launcher was updated. Now the launcher will either work with Fabric running Java 8, or Vanilla running Java 15, but not both. Also seeing two slightly different behaviors depending on how the launcher is configured:

• If I set "keep the launcher open while games are running" and "open output log": The game hangs on "preparing..." then hard crashes

• If I do not have the above settings on, the launcher just hard crashes

Output logs do not state anything useful from what I've seen; they don't even update on the crash.