The final step was that I opened too many files in Aseprite, and then closed the software without closing the tabs. When I reopened it, I got this error and couldn't open it anymore. Reinstalling didn't work either.
Sorry for the inconvenience. We will fix this soon.
Meanwhile try this:
Right click on the software on steam > Properties >Betas , click the dropdown then click “beta - BETA -NEW FEATURES”. Try to launch again.