diff --git a/kallithea/lib/utils.py b/kallithea/lib/utils.py
--- a/kallithea/lib/utils.py
+++ b/kallithea/lib/utils.py
@@ -371,6 +371,7 @@ def make_ui(repo_path=None):
         else:
             log.debug('hgrc file is not present at %s, skipping...', hgrc_path)
 
+    assert baseui.plain()  # set by hgcompat.monkey_do (invoked from import of vcs.backends.hg) to minimize potential impact of loading config files
     return baseui
 
 
diff --git a/kallithea/lib/vcs/utils/hgcompat.py b/kallithea/lib/vcs/utils/hgcompat.py
--- a/kallithea/lib/vcs/utils/hgcompat.py
+++ b/kallithea/lib/vcs/utils/hgcompat.py
@@ -2,6 +2,7 @@
 Mercurial libs compatibility
 """
 
+import mercurial.encoding
 import mercurial.localrepo
 
 
@@ -11,3 +12,6 @@ def monkey_do():
     mercurial.localrepo.localrepository._lfstatuswriters = [lambda *msg, **opts: None]
     # 3.5 7699d3212994 added the invariant that repo.lfstatus must exist before hitting overridearchive
     mercurial.localrepo.localrepository.lfstatus = False
+
+    # Minimize potential impact from custom configuration
+    mercurial.encoding.environ[b'HGPLAIN'] = b'1'