Unable to process your request / external link

Unable to process your request / external link

Hi,

We have extensive notifications one of which informs the opportunity owner of any changes made. Thus includes a direct link to the opportunity.

This used to work fine but today a report arrived; we tested and confirmed that indeed links no longer work and below message is displayed on CRM page. 
Unable to process your request.
Click here to go back.

The link in the email template is; orgid is replaced with xxx for security.
https://crm.zoho.com/crm/orgxxxxxxxxx/tab/Potentials/$%7BOpportunities.Opportunity%20Id%7D/

Any advice to correct the error would be appritiated.

Regards,
Burak