@chfour Rarely, if you set up correctly (but Element's UI guidance does need improvement). I use key backup (no manual import/export), my sessions are verified and kept alive, and I don't use matrix.org. For me, causes, in descending frequency, are:
- toggling the verified session requirement without understanding,
- the opposing homeserver has setup issues,
- the opposing homeserver experienced an outage and MegOlm session is disrupted.
See https://joinmatrix.org/guide/fix-decryption-error/ for troubleshooting guide.