Agent failing to switch protocol (426 Upgrade Required)

G'day, I'm having issues to connect the agent with my Gitlab installation. The logs of the agent pod are the following:

{"level":"info","time":"2021-12-21T12:52:44.339Z","msg":"Feature status change","feature_name":"tunnel","feature_status":true}
{"level":"info","time":"2021-12-21T12:52:44.339Z","msg":"Observability endpoint is up","mod_name":"observability","net_network":"tcp","net_address":"[::]:8080"}
{"level":"warn","time":"2021-12-21T12:52:44.369Z","msg":"GetConfiguration failed","error":"rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing failed to WebSocket dial: expected handshake response status code 101 but got 426\""}
{"level":"error","time":"2021-12-21T12:52:44.370Z","msg":"Error handling a connection","mod_name":"reverse_tunnel","error":"Connect(): rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing failed to WebSocket dial: expected handshake response status code 101 but got 426\""}
{"level":"error","time":"2021-12-21T12:52:44.370Z","msg":"Error handling a connection","mod_name":"reverse_tunnel","error":"Connect(): rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing failed to WebSocket dial: expected handshake response status code 101 but got 426\""}
[...]

If there was a way to see the response body, I might would have gotten a better understanding of what's causing the issue, but I couldn't find any documentation on how to configure the agent, to see if I can toggle on verbose logging or anything alike.

I couldn't find anything on this issue in the official docs, I'm not really sure how to approach this issue.