diff --git a/changelog.d/6682.bugfix b/changelog.d/6682.bugfix
new file mode 100644
index 0000000000..d48ea31477
--- /dev/null
+++ b/changelog.d/6682.bugfix
@@ -0,0 +1,2 @@
+Fix "CRITICAL" errors being logged when a request is received for a uri containing non-ascii characters.
+
diff --git a/synapse/http/site.py b/synapse/http/site.py
index 9f2d035fa0..911251c0bc 100644
--- a/synapse/http/site.py
+++ b/synapse/http/site.py
@@ -88,7 +88,7 @@ class SynapseRequest(Request):
     def get_redacted_uri(self):
         uri = self.uri
         if isinstance(uri, bytes):
-            uri = self.uri.decode("ascii")
+            uri = self.uri.decode("ascii", errors="replace")
         return redact_uri(uri)
 
     def get_method(self):
diff --git a/synapse/logging/context.py b/synapse/logging/context.py
index 33b322209d..1b940842f6 100644
--- a/synapse/logging/context.py
+++ b/synapse/logging/context.py
@@ -571,6 +571,9 @@ def run_in_background(f, *args, **kwargs):
     yield or await on (for instance because you want to pass it to
     deferred.gatherResults()).
 
+    If f returns a Coroutine object, it will be wrapped into a Deferred (which will have
+    the side effect of executing the coroutine).
+
     Note that if you completely discard the result, you should make sure that
     `f` doesn't raise any deferred exceptions, otherwise a scary-looking
     CRITICAL error about an unhandled error will be logged without much