I've got the exact same problem. I tried all browsers, always with the latest extension. I'm using Arch Linux as well and the browser extension for Enpass 6 beta worked for version 6.0.0.105 until the extension update yesterday. (version 6.0.0.149 had the same problem for me)
I tried to force Enpass to use another port than 10391, however, the problem persists. In this case TCP packages to 10391 are actually sent.