This issue is related to permission. The Client profile is not allowed to view "Changes". What you can do is give them the view and modify all permission. Not the best scenario, but that the problem. This profile is missing the visibility of the change pages and controllers.
To test this; Assign the Change Manager profile to this user and you will see all is working.
Probably better is to create a Service Request to approve instead of an Change. That way you don't have those issues and your client can still approve all.
What we did with one of our customers is related to custom code where we share the record with the assigned approvers, but that's pretty complex.
Hope this helps..
Just now found a wierd thing happening,
The error was coming when there were no values in the Impact and Urgency fields, when I set some values in these fields like "LOW, LOW" there was no error.
When I removed the values in these fields and set it to -None- it gave me the same error again.
Is this a OOB behaviour?
Are these required fields, or included in Validation rules?
Sounds like those are required
Nope, this is not included in any validation rules.
And the exception is thrown by a OOB trigger.
In that case best to raise with BMC support. They can check in your org when you grant access!
Ps. Have you tried activating deburg log, reproduce issue and check log files?