A problematic add-on, gone wrong, caused it, however, trying to run the process just keeps causing the same error to appear, thus getting nowhere. I have disabled the add-ons, but still can't get past it -- thus the issue is that I can not install other stuff I have to do because the system is stuck.
Do the problem add-ons have to be removed, not just disabled, to get rid of it? Or is there a method I can use to cancel any such processes?
Generally it should give you the option to cancel it, but if it's hitting an error immediately, it may not end up being exposed. In that case, unfortunately it likely needs to be removed in the database directly (a single row for it in the xf_deferred table).