Googled your error message and found quite a few people with the same issue, most from a couple months ago.
Someone was able to fix it by doing this:
“I manually removed the files from my computer to fix this issue - not just uninstalling or removing the build, but going into my Program Files folder (and others) and deleting RotMG files. The error code is supposed to mean that your game files aren’t where they’re supposed to be. They were exactly where they were supposed to be for me, so I got rid of them and then reinstalled and it worked” (Source)
Here’s a video about this issue.
Someone on a Steam discussion thread was able to fix it like this:
" im not sure if anyone is still having this problem, but anyway, if you are. the fix I found is going into the launcher, pressing on the settings button in the top right and then clicking on “remove build” from here, you can then reinstall the build by pressing the play button again and it should work.
open launcher on steam >> click settings >> click remove build >> click play
on a side note, you may have to press play twice as sometimes it doesn’t register." (Source)
Someone else in the same thread got a reply from DECA support with this message and it worked for them:
" Hey guy I just talked with DECA support,and i fixed it you to do following this step first you. *Open regedit (WIN+R, type regedit, hit enter)
*Navigate to ‘Computer\HKEY_CURRENT_USER\Software\DECA Live Operations GmbH’
*Remove all subfolders EXCEPT for the master folder itself - > DECA Live Operations GmbH"
Have you tried sending a message to DECA’s support team yourself? Goodluck!