It sounds like your "Managed Systems" query is including machines that don't have the tag your trying to clear. If that's the case then I guess it's failing because there is nothing to clear. Try adding "has tag" to your query filter so that only the machines that have the tag your interested in are included. If I'm right and you have more than one tag your trying to clear you may need to create queries and sub-actions for each tag. Hope this helps.
I've run into this same issue, my query is definitely returning systems that have the tag I'm trying to clear. If I change the tasks to simply add a tag, it still fails but as 'wupe84' stated, there is no detailed information as to why it failed. Anyone have any thoughts...? I'm running ePO 4.6.5 (Build 168). Thanks,