I tried exporting a backup of my electrum wallet on Android and import it into electrum on PC but it didn't work. I assume there is a compatibility issue.
Out of curiosity... where did Electrum export your wallet backup to? Or were you using a copy of the 'actual' wallet file that you exported from the protected app folder using 'root' permissions?
When I select the "Export Backup" option on my phone it doesn't seem to do anything. I get no notification/'toast' message to say it's been exported, but I don't get any error messages either, it just switches back to the default wallet view (with Send/History/Receive tabs etc).