Reprocessing while Reprocessing breaks Reprocessing #75723
Description
Starting to reprocess an issue creates a new group (aka issue). All old errors will slowly be reprocessed and moved into the old group. But during this process the user can trigger another reprocessing on the newly created group, which will interfer with the currently running reprocessing and break it.
We need to investigate this bug further and either prevent reprocessing while the old one is still running or handle this case gracefully (probably latter).