A bug in parsing certain p2wsh-p2sh transactions was fixed in this new version of Electrum 3.0.6. The bug affected non-multisig p2wsh-p2sh transactions like those sent by an exchange called BitPanda.
Users affected by this bug experience a perpetually synchronizing Electrum client. On the history tab the transaction affected by the bug will display +0 in the amount column. If they run Electrum via the command line with the -v verbose switch they see a message saying that Electrum “cannot deserialize transaction”.
You can download the latest Electrum release and install it to fix this issue.