I think the server error is simply because your replacement of transaction doesn’t meet the server policy. So start by changing the server, check the settings and see if the server is on automatic and the switch to manual and select from the list of servers there.

But before doing so try to look at the second error of insufficient funds which is that the server needs the total transaction fee of the transaction you’re replacing must be lower than the new transaction fee. This server might be using BIp25 rules which is the fee to be payed for replacement of transactions must be higher than the total fees of all transactions to be replaced in the mempool.

So consider paying much higher fee, if it doesn’t works then go for change of server