I can tell you that JEMH 0.9.8-1 definitely contains the fix for the classloading issue.
I would NOT dig up the old class and put it in place as suggested by support, if I had your issue. This will just cause support to tell you that what you did is not supported and possible make it harder to work other support cases.
If you open the jemh-mail-handler-0.9.8-1.jar, find the class com.javahollic.jira.emh.processor.CSVFieldProcessor and open it with something like jdgui ( Java Decompiler), you will see the following import:
Noticed it is not trying to import: com/mindprod/csv/CSVReader
The version of JEMH just before this one, does not have the updated import.
I suspect either you are on the previous version of the jar or maybe when you tried to fix the issue, you left the previous JEMH jar file in the classpath. Since the jars have the version number in them, it would be easy to accidently leave the old version in place when you copy the new version in.
BTH - just an Atlassian customer!