I have processed same issue recently regarding the topic, so post it here in case you face it.
Backend error occurred (Received “failed to fire action: argument not found” …) on webgui in-placed in Portal.
The issue could be reproduced when you hide/expand the side panel of Portal by the expand navigation button to resize webgui,
see following picture for reference.
Following screenshot shows the detailed error message.
The issue is documented in SAP Note 2195859 SAP GUI for HTML: “failed to fire action: argument not found” on resizing.
Correction is with a later Kernel patch, see the Note for detailed patch level.