Update: If you have a transaction that is stuck as pending you can export the private key of one or more inputs and sweep it into another wallet. Doing so will invalidate the pending transaction in Electrum.
Some have had problems sending Dash from Electrum after the update to Dash 12.1.
A common error message is:
Your client produced a transaction that is not accepted by the network any more. Please upgrade to Electrum 2.5.1 or newer.
If you see this message, the temporary fix is simple. Update your network connection to electrum.dash.org. To update the network, select Tools => Network.
In the Network window, uncheck Auto-connect and manually enter electrum.dash.org or electrum.masternode.io as the server.