After migrating phones, you may get an "Invalid Authentication" error when signing a transaction.
Did you recently changed phones, migrating your data from your old to new phone using automated data transfer or (iCloud) backup restore?
For security reasons we bind the encryption in XUMM to the device, so if someone gains access to your iCloud backup, one cannot restore your wallets and steal your funds.
The solution is to remove your accounts from XUMM (Settings » Accounts) and add (import) them again from their secret/mnemonic/family seed.
XUMM 0.6.1 and newer, to be released in Decemer 2020, will add a warning message on the new phone on first start with more information, so this won't be a surprise when signing for the first time.