Every day brings a new surprise in the life on consultant. And today was also not an exception.
We had some SSRS reports for our client which required some changes in the default values of multiple parameters for each of the reports. So our SSRS report developer did all the changes which was pretty quick for him. He tested in BIDS and everything worked fine.
Then once I started uploading the reports and viewed each of them, I could see that the default values selection was empty. My initial thought was that there were some glitches from the report side (I should not have thought like that considering how good he is, but still you know humans 
). I went up to his desk and he showed me how each report is working fine.
Then I realized that it had to do something to do with CRM. I searched some blogs and it suggested restarting Reporting services and all.
Finally we had no other option to delete and re-create the reports. After that everything started working fine.
Please note that before you delete a report, take a note of the all the sub-reports which were referencing it. Because once you re-create the report, you would need the sub-reports to point back to the parent report.
I am sure there must be some better way to get rid of this error. However we had a deadline to meet for our QA team and could not afford to do much R&D. So if you are facing the same issue, deleting and re-creating the report might do the trick for you.
Hope this helps!