I'm getting this error in a Commandbox task runner where I call cache.reap() right before using the cache. It seems the reap is removing the metadata from the indexer as I'm trying to use it. The code in the task runner is just running a getOrSet().
Check and see if the same issue exists with the new async manager method of reaping. In my case, I had the reap freq set to 1 minute and the item I was getting from the cache timed out in 1 minute, so lots of opportunities for it to time out while it was being accessed.
The metaadtaindexer needs to account for the possibility of the metadata disappearing from the hashmap while it's accessing it.