are you pasting the whole line from the wallet file? like this:
"xprv": "xprv9uEDMxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxcDp3boP"
or are you just copy/pasting the actual text... NO " marks etc: xprv9uEDMxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxcDp3boP

NOTE: it should be just the actual text: xprvblahBlaHblAH
HCP, we meet again

I have the same problem: the xprv seems not to have a normal format even though the file is not encrypted. It does not start with xprv, but instead contains "+" or "/" in between and ends with "="
Interesstingly, the x1/ and x2/ private keys are different with every electrum version i use even though I'm using the same seed words to restore the walletevery time.
EDIT: I'm a fucking idiot: I always created a unencrypted wallet, but still set a password. Leaving the password field empty solves all problems.