I have processed same issue recently regarding the topic, so post it here in case you face it.
Description:
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.
Solution:
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.