I faced a similar issue and found this reddit post quite helpful. I'm not a bitcoin expert but I will write an answer describing what I think is happening, since there are no other answers yet.
When you sent the first low-fee tx, it got propagated fine to the network but remained unconfirmed. When you opened the Electrum wallet a few hours later, this unconfirmed tx disappeared because your electrum wallet connected to a node which removed your tx from the mempool for being below its minimum relay fee.
So you created a second transaction, but while most nodes (including block explorers, miners) saw your transaction, they immediately ejected it out of the mempool for being an attempted double spend.
Only your first tx has any chance of going through. But if it has very low fees it might not be mined and be removed from all mempools after some time (3+ days?). So, you have to wait for the first transaction to be removed from all mempools and then create a new transaction with higher fees.
Any confirmation or correction for the above from an expert would be appreciated..
Here is what electrum documentation proposes for cases of unconfirmed transactions:
If you have made a transaction that is unconfirmed, you can:
- Wait for a long time. Eventually, your transaction will either be
confirmed or cancelled. This might take several days.
- Increase the transaction fee. This is only possible for
‘replaceable’ transactions. To create this type of transaction,
you must have enabled ‘Replace by Fee’ in your preferences, before
sending the transaction.
- Create a ‘Child Pays For Parent’ transaction. A CPFP is a new
transaction, that pays a high fee in order to compensate for the
small fee of its parent transaction. It can be done by the
recipient of the funds, or by the sender, if the transaction has a
change output. (Here is a step by step guide for this option: link)
