"""""""I removed back that MAXDOP option and rather created clustered index on the temporary table (I know it should have been there in the very first place). Now it worked perfect. But my quesiton is why the issue arouse for that one day only ? There is no 'WITH RECOMPILE' option for that procedure either. It worked perfectly fine before that day and after that day. There was no drastic change in any of the involved table's volume."""""""

As I said in the above quotes, the issue got cleared after creating clustered index on temporary table. so that might be it but I have one doubt here:

"Why it was running perfectly without any issues before and after that one bad day ?"

Secondly, when once again I removed the clustered index and ran the query, it started working fine even without clustered index. So other doubt is:

"What got changed after I created the clustered index on temporary table for once (even after trying dropping clustered index and/or dropping and recreating that temporary table) ?"