Hi
Sorry to join in on this thread but I am having a similar issue.
I am using Electrum 2.5.4 and until last week it was sending transactions without any issues. The past few days however I keep getting an error screen also with the following message in part:
error: The transaction was rejected by the network rules.(66: insufficient priority)I first took this to mean that the value of what I was trying to send was too low, but now I am still getting the same error with much larger amounts (BTC not mbtc).
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
I used to have Dynamic fees selected which was only running at about 0.00015BTC/kb, but even turning that off and going to 0.0005BTC/kb as I have seen recommended in other places does not allow the transaction to go ahead
Thanks in advance to those that might be able to help out.