Hi
If you're having issues with CL_WAPI_MOBILE_INBOX errors when running the Fiori Purchase Requisition Approval (or other approvals), check you've installed ALL the pre-requisite SAP Notes (even if SNOTE says the note is not applicable for your release!)
In our case, we could not install SAP Note 1805986 but this was a required note for SAP Note 1774246.
Also always check to see you have the latest version of the SAP Notes installed if you have errors.
See related blog and comments...
Hope this helps
Paul