I never got that error, but searching in on the Web this StackExchange thread appears for an old Bitcoin version.

It is possible that you are moving too many "inputs" at once (inputs are created always when you receive coins, from transactions, mining or staking). Have you got many transactions, e.g. from mining or staking and want to move them all at once?*

Try with a smaller amounts, e.g. if you have got many transactions of 10-15 SLM (like they're common for mining)  try to move them in sets of 10000, and if this still doesn't work, of 1000 or even 500 SLM.

*(Explanation according to stackexchange, which requires a little bit of knowledge: this happens when the transaction gets too big. For every input there are about 50-100 additional bytes. The limit for Bitcoin is 100 kB, but I didn't find the limit for Slimcoin).