update form troubleshooting

src/essentials-for-yootheme-pro/addons/forms/troubleshooting.md

@@ -4,6 +4,12 @@ Learn how to handle common issues within essential forms addon.
 
 ## Submission Errors
 
+### Submission Fails if Cache is Enabled
+
+The form displays an _Internal Server Error_ when page caching is enabled. When the cache is cleared, it works, but soon after, it stops working.
+
+This is expected. Due to the nature of how forms validate submissions, pages containing a form cannot be cached. Find a way to make a caching exception for those pages.
+
 ### Submission Fails With Vague Error
 
 The form displays a _Submission Failed_ error without any further details, it is most likely a suppressed PHP error. To get more insights about it you can: