documentation: mention possible update error (#44)

* documentation: mention possible update error

* Update README.md

* wording fixes

* fix missing double whitespace

Author: EvilOlaf

README.md

@@ -90,6 +90,19 @@ environment:
 Images tagged as `latest` are **tested and known to work.**[^3]
 Any other tag represents active development and/or automated **untested** builds.  
 
+### :question: Game and server version don't match / cannot login
+
+> [!NOTE]
+> This is typically caused by a long-standing SteamCMD issue (affecting various games since 2015) where updates silently fail.  
+> Not specific to SCUM or this container.  
+> Check your logs for an entry like this: `Error! App '3792580' state is 0x6 after update job.`
+
+**Workarounds that have helped others:**
+- Restart the container multiple times until the update succeeds
+- Delete `appmanifest_*.acf` files in `./scumserver-data/steamapps/` and retry, forcing steamcmd to validate game files
+- Last resort: back up `./scumserver-data/SCUM/Saved/`, wipe game files, let SteamCMD perform a fresh
+
+
 ## :information_source: Footnotes
 
 Some port exposures are unnecessary. However, I could not find clear documentation which ports and protocols are actually required. Exposing additional ports/protocols won't cause harm.