How can we found exact cause for this abend like who purged the task and whether due to file data or not?
Look at the CICS message log or system log. You could also specify NOWAIT on your READNEXT command and see if you get a non-zero EIBRESP back on the READNEXT.
The link you referenced is for an IBM fix to a particular problem. You would have to contact someone in your site's support group to determine whehter or not the specific condition mentioned in the link applies to your site. If you do not know how to find and read the CICS message log or system log, then you need to contact your site support group anyway for help since the problem is one specific to your site and hence there's very little, if any, chance anyone on this forum could help you.