If it persists over restarts - it's rare but possible that the blockchain_headers file got corrupted or it downloaded corrupted chunks. Please close electrum, find the blockchain_headers file, delete it and restart. It'll re-download it which will take a moment (20 MB) and then ideally it should fix your problem.
Oh this may be dangerous. I once tried to restore an electrum.dat file without the blockchain header files and it didn't work. Note this was not a backup made using the UI but a straight copy of the file from the .electrum folder. So before you go deleting the header files do a proper backup of your wallet using the menu options in the UI.