Does this always happen with the same wallet? For whatever reason some of the outpoints are empty. Use the coin control feature to try and isolate the culprits and report here.
Thanks Goatpig,
You're right, the address in most of my funds are deposited is bugged, I just sent a test transaction to the wallet and tried to transfer the same coins after (using only this test address with the
coin control) and the transaction seemed to work.
The problem is that literally 99% of the funds are in this bugged address...