We've just encountered the same error after updating to 2.55.5010.521 - we tracked it down to an '&' in the path name. Moving the .pbix to a different folder opened fine, but then I moved it into a subfolder /A&B and it failed. So try moving the .pbix to a different path and see if that helps.
As a temp fix you can try disabling the optimized FileSave:
1. Close the PBIDesktop application 2. Control Panel>System>Advanced system settings>Environment Variables 3. User variables... New... Variable name: PBI_fastLoadSave Variable value: false 4. Start PBIDesktop and try to open file
(You'll need to remove the environment variable once you install the update)