Good to hear DStefanov and thanks for confirming.
Can you perhaps explain dEBRUYNE why in this case being connected to a remote node has a different result than being connected to your own?
The amount of inputs a transaction can use is limited in case of utilizing a remote node (I do not remember the exact reason off the top of my hat). No such limit is in place in case of using a local node.
Note that the threshold is quite high and is typically not reached by a user, especially after RingCT was introduced.
Didn't the error message he got say something about --restricted-rpc? Could it be that if the remote node runs with --restricted-rpc option then the sweep unmixable doesn't work?