That's the same warning message I came here about.
Booted up Linux Mint 19.3 this morning and the Update Manager indicated there was an update for Enpass from 6.3.3.601 to 6.4.0.631. I opened the Enpass application, clicked the hamburger menu > Help > Check for Updates, and saw the message "Your software is up to date." That's when I decided to check it out on the web page, https://www.enpass.io/, and saw the following warning messages:
Warning: session_start(): Cannot start session when headers already sent in /home/dvinod/webapps/enpasswebsubscription/wp-content/plugins/helpful/core/classes/class-helpful-helper-values.php on line 158
Warning: Cannot modify header information - headers already sent by (output started at /home/dvinod/webapps/enpasswebsubscription/wp-content/themes/enpass/header.php:1) in /home/dvinod/webapps/enpasswebsubscription/wp-content/plugins/helpful/core/classes/class-helpful-helper-values.php on line 162
Enpass
So, I'm waiting until I get some credible assurance that it's OK to update...or not.
P.S. Using Firefox 73.0 (64-bit).