Using polymer 0.3.0-rc.2+1.19.3-rc3 and memoryleakfix 1.19.3-0.7.0 causes the game to show an error screen upon joining a singleplayer world, however no error information is found in the Minecraft log.

Screenshot: https://files.lostluma.dev/W1q49D.png
Minecraft Log: https://files.lostluma.dev/NhZCDP.txt

Previously using the 1.19.2 versions of these mods worked fine.
Opening this issue was requested by patbox, see Patbox/polymer#51 for the issue on the polymer repo.

This has been fixed in Patbox/polymer@9709f18.