My team has this problem quite often. We do not know the cause but usually what works is power cycling, replacing all batteries (including 9v), copying the code, closing Robot C, reopening robot C and pasting the code into a new file. This works every single time for us. If it still doesn’t work I suggest adding a check for updates step. Good Luck.
A simpler fix is to just re-upload firmware.
I can confirm we were using a linearizing motor value remapping array.
We would get “Flash write failure” error codes, then after several failures, we would get the Slave cpu error.
Moving the VEXnets very close to each other seemed to slightly improve the issue, but maybe that was psychological.
I will try your fix. Thanks @jpearman