Bugfix: Missing UI_SERVE_BASE_URL lead to incorrect asset urls (#11628

)

src/prefect/server/api/server.py

@@ -370,7 +370,7 @@ def create_ui_static_subpath():
         replace_placeholder_string_in_files(
             prefect.__ui_static_subpath__,
             "/PREFECT_UI_SERVE_BASE_REPLACE_PLACEHOLDER",
-            base_url,
+            base_url.rstrip("/"),
         )
 
         # Create a file to indicate that the static files have been copied