BTW: Since this seems to be the necessary workaround for now, recentlyish I wrote a PowerShell script to make backing up a working bing.attribution.xml and later restoring it when JOSM is giving the error again easier. Usage is .\attribution-backup.ps1 backup and .\attribution-backup.ps1 restore, and you can add the parameter -DryRun to get the output describing what would be done without actually touching any files. I also have instructions in a bit more detail in a comment on the Gist:
At a time when the Bing imagery is working, run
.\attribution-backup.ps1 backup. Then, at a later time when you get tiles stuck on “Error: Attribution is not loaded yet”:
- Close JOSM.
- Run
.\attribution-backup.ps1 restore.- Reopen JOSM.
- Before loading the Bing imagery layer, disable cache updates by opening [File] > [Work Offline…] and choosing [Cache updates]. This prevents JOSM from trying to overwrite your working Bing attribution file with a non-working one again.