SL360 - Reopening saved files always corrupted

I'm a strong SL user, using it daily for the last 15 years. New problem has just appeared.

When I open an existing project, make some changes, then save it under a new name, it cannot be reopened. Error message says file is corrupted. If I try to open it a second time, it works.

Has anyone seen issues even vaguely similar to this?