Hi Paneendra,
I had similar issue. My customer could export 4 applications, but only 1 application was failed to export.
In that case, database had inconsistent records. Updated the records, and then the problem was resolved.
Could you check following query? Normal behavior is no records. If you have one or more records, we need to correct the records. If so, I recommend to open a support case to fix them.
select * from executable_instances where delegate_id NOT IN (select id from executable);
Thanks
Yas