It seems to be in the interactions with other bots (i.e. them standing in the way, I guess), as this does not happen when I put him into his own little area with nothing but the two required storages, the furnace and fences all around...
Will try to find a minimal setup where the problem occurs.
(Also, I can now confirm that bots that are unreachable for the recharge bots get charged remotely^^)
Okay, it seems to be definitely related to other bots blocking their way:
I've now got a setup where he skipped the step of actually walking to the furnace and jumped straight to trying to insert the item!
http://s000.tinyupload.com/index.php?file_id=72337184487671410134