diff --git a/vcsserver/scm_app.py b/vcsserver/scm_app.py
--- a/vcsserver/scm_app.py
+++ b/vcsserver/scm_app.py
@@ -107,6 +107,18 @@ class HgWeb(mercurial.hgweb.hgweb_mod.hg
         return super()._runwsgi(req, res, repo)
 
 
+def sanitize_hg_ui(baseui):
+    # NOTE(marcink): since python3 hgsubversion is deprecated.
+    # From old installations we might still have this set enabled
+    # we explicitly remove this now here to make sure it wont propagate further
+
+    if baseui.config(b'extensions', b'hgsubversion') is not None:
+        for cfg in (baseui._ocfg, baseui._tcfg, baseui._ucfg):
+            if b'extensions' in cfg:
+                if b'hgsubversion' in cfg[b'extensions']:
+                    del cfg[b'extensions'][b'hgsubversion']
+
+
 def make_hg_ui_from_config(repo_config):
     baseui = mercurial.ui.ui()
 
@@ -156,6 +168,7 @@ def create_hg_wsgi_app(repo_path, repo_n
 
     baseui = make_hg_ui_from_config(config)
     update_hg_ui_from_hgrc(baseui, repo_path)
+    sanitize_hg_ui(baseui)
 
     try:
         return HgWeb(safe_bytes(repo_path), name=safe_bytes(repo_name), baseui=baseui)