Looks like there are some additional stake rules for Decred that Paymetheus isn't following. The error is being hit when attempting to publish the transaction to the network through dcrd, which is doing full validation of the tx. I'll open a GitHub issue since this is a bug in the software.
I did some more investigation here and the issue is not directly with Paymetheus (even though it does construct the tx) but in the wallet itself because it is providing unspendable outputs to Paymetheus to use for the tx. We are trying to figure out why it is doing this. Did you use a ticket/voting address from this same wallet for a ticket purchase?
Narrowed the issue down to this: https://github.com/decred/dcrwallet/issues/529 However, i'm still interested in whether you used one of your own addresses from that same wallet as the voting address for a ticket purchase. That is not supported at all since Paymetheus doesn't vote.
There are a couple issues that we need to get resolved here. I'll try to explain them and then we can decide how you want to proceed in order to get your wallet back into a usable state. You bought tickets with voting rights assigned to one of your own Paymetheus addresses. This is not a supported operation since Paymetheus does not issue vote or revocation transactions when the ticket is called upon or missed. Instead, correct use of the software would purchase tickets for a voting wallet or for a stakepool. We could have improved the UX here by erroring when trying to purchase tickets that pay to one of your own wallet's known addresses, but in this latest release this is not done, hence the situation you are finding yourself in. The original issue you hit of not being able to send a transaction is caused by a bitcoin-ism in the dcrwallet gRPC server implementation that considers all unspent outputs as spendable. In Decred this is not the case, as some kinds of outputs are only spendable by transactions in that same "tree". This prevents spending stake transaction outputs from regular transactions or regular tx outputs from stake transactions. When Paymetheus asked for the wallet to provide outputs that it can use to construct a transaction, wallet erroneously included some of these stake transaction outputs. While they are unspent, they are not usable for your transaction. Fixing this first issue is a simple code fix, and we can get that into your hands rather quickly. There is also the second issue of now having tickets in your Paymetheus wallet. Without some manual intervention, these funds are essentially stuck. Paymetheus will not vote if the ticket is called, and will not even create a revocation after the ticket expires. This is due to both of these operations requiring dcrwallet to run in a special mode to create these transactions, and to keep the wallet persistently unlocked so that it can sign the transactions. To get yourself out of this issue, we can't at the time provide a simple code fix and you will need to use the command line tools to open up some extra options that aren't available in the Paymetheus GUI. Some options: Import your Paymetheus wallet's seed into a new command-line dcrwallet, run that with the staking mode enabled, and unlock the wallet using dcrctl so it can produce votes/revocations. Start the standalone Paymetheus (not the dcrd+Paymetheus launcher!) with some additional command line arguments that enable the JSON-RPC server and run it in staking mode. Then use dcrctl to unlock the wallet. Out of those two options, I prefer the first, as this will let you run two wallets simultaneously and you should not see any issues with Paymetheus inadvertently locking the wallet and breaking voting. I will begin working on a code fix for the first issue, but please let me know what you want done about the second one. Since it requires manual intervention, I would prefer to help you do that with some kind of realtime communication, either over IRC or Slack, instead of using this forum.