Easy way to reproduce:
go to a dashboard
click the Actions button for one of the issues
select Close Issue
In the popup, click close
Oops - an error has occurred
Looking better at the error message, it seemt that the error originates from the JFS plugin (by Quisapps):
at com.quisapps.jira.fieldsecurity.filter.processor.IssueEditRequestProcessor.parseMultipartRequest(IssueEditRequestProcessor.java:588)
at com.quisapps.jira.fieldsecurity.filter.processor.IssueEditRequestProcessor.filterMultipart(IssueEditRequestProcessor.java:256)
at com.quisapps.jira.fieldsecurity.filter.processor.IssueEditRequestProcessor.filter(IssueEditRequestProcessor.java:106)
at com.quisapps.jira.fieldsecurity.filter.FieldSecurityFilter.doFilter(FieldSecurityFilter.java:113)
This plugin is atm enabled for Operations projects, but also for some Transitions projects (why?). First I disabled JFS only for HEIRO, but I still got the same error.
Next I temporarily disabled the JFS plugin completely and after that the error didn't occur anymore.
Finally I enabled the plugin again because we need this plugin.