If it is WOW51900319 Error Code you all getting - I have that too - https://eu.battle.net/support/en/article/112213

Some have there Max background FPS set to 30 and Max Forground FPS set to 60 - and that fix the problem for them.

Isboxer do overrite this setteings and set them back to 0 in your .WTF file if you let Isboxer control your FPS on swap.

SET maxfps "0"
SET maxfpsbk "0"

(Are they new as they are last in my .WTF file and never seen them before ?)