Sketch introduced a Safe Mode feature when they released version 51. If Sketch crashes, the next time it is opened, it’s in Safe Mode, meaning plugins are disabled.
Could Not Connect to Abstract Plugin message should resolve if you enable the Abstract plugin in Sketch again (Sketch > Preferences > Plugins). You can also restart both Sketch and Abstract and this should re-enable your plugins.
When Sketch crashes, it could be Abstract, Sketch, or another Sketch plugin.
You can send a crash report to our Support team for help. Use Spotlight to search for this folder and share crashes specifically related to Sketch or Abstract: