In the 7.1 update we updated the encryption utilities to use the openssl library purely over mcrypt as mcrypt is being deprecated in PHP 7.1 and completely removed in PHP 7.2. Unfortunately this means that the previous encryption keys that were generated with mcrypt can no longer be used and therefore the app must generate new keys. Your email credentials (along with other credentials such as Google Integration) must be re-entered so that the newly generated keys can encrypt them. Once the new credentials are added, everything should work as normal.
Thank you for the update.