I took a look into this issue and decided to upgrade devenv to the new major version (the lines which cause the exception are not there anymore in devenv 2.x).
So it seems to be running all the processes as intended but the frontend will say "cannot connect / try again" which might very well be an unrelated CORS or TLS issue.
I would love to start a PR, but this repo does not allow outsider PRs.
In case someone wants to try it out, here is the commit:
https://github.com/fluxerapp/fluxer/commit/2af1d34c711c2dd312046ea20755c0f8b0daabc2
Thread
Comment by @maxhille