Access to view the subscription was denied when trying to move a thread.

I'm able to reproduce this. Logging a bug. In order to workaround, a user with Manage Forum permission (as well as Edit in the source forum and Create in the target forum) need to move the thread.

Hi  

 the workaround requires the 'manage forum' permission - we can't give this permission to moderators or they could delete the forums

Luke R Davidson  

Fixing the error will require a code change. A different/elevated user might need to manage forum thread moving for now.

If we wanted to develop a custom plugin to fix this issue...do you have any ideas on what would be needed or could point me in the right direction?

The error occurs within the internals behind the call to the content conversion API - so you would need to rebuild the conversion logic from scratch, which isn't really feasible based on the amount of internal logic being done.

Something to investigate would be creating a new ISecuredContentConverter for thread-to-thread conversion, and wrapping a call to the content conversion API with a permission grant/ungrant - that is, programmatically add the permission, call the conversion API, and then revoke the permission again. You will need to disable the out-of-the-box Forum Thread Mover plugin to ensure your new converter will be used.