I got this error, if I do an update to 1.8.6, with 1.7.2 it works.
In 1.7.2 there is also a message:
Info: Firmware Development Build
Your printer's firmware is a development build of Marlin (build date 20211105). It might be more unstable than a release version and should be kept up-to-date.
That sounds like the ingress issues with that one add-on the author and I have been trying to debug on Discord in #dev-misc for weeks now, but due to completely async communication with not much progress so far.
In short: something in this whole ingress config causes issues with OctoPrint's CSRF protection or more strict reverse proxy config requirements present in 1.8.3+. Until those are resolved, accessing OctoPrint through a Home Assistant ingress reverse proxy is likely not a good idea.
Is this still an outstanding issue, because I believe I have this issue. Is there anything in the mean time while it is being fixed that I can to resolve the issue? Is there anything I can provide that will assist in getting the issue resolved?