The user can open a diagram in the Editor and the Collaboration Hub, but he can't open the diagram in HubNext, it is just an empty page.
Check if your profile has a language set up:
Open the Explorer -> Setup -> My Profile
If the language is empty, choose one, save and try again.
If the issue persists, please contact the support team at email@example.com or create a ticket in our SAP Support Launchpad and reference the ticket number SPM-2411.
KBA , BPI-SIG-HUB , SAP Signavio Process Collaboration Hub , Known Error
SAP Process Collaboration Hub by Signavio all versions