User picker failed when having a large user list in the project role

When adding a large list of users (up to a few 100 users) to the project role, the user picker will be failed throwing the error message "The server is temporary not reachable" (like in the screen shot). The root cause could be that the header of the HTTP request exceed the default header size limit ( maxHttpHeaderSize by default set to 8192) defined in the file server.xml.

Workaround #1
Increase the maxHttpHeaderSize in your Tomcat configuration:

• Edit $JIRA-installation-directory/conf/server.xml file
• Find the connector tag used to configure connections to JIRA, if you're using the default server.xml configuration, it should look something like this for example:

  <Connector port="8080" maxThreads="150" minSpareThreads="25" connectionTimeout="20000" enableLookups="false" maxHttpHeaderSize="8192" protocol="HTTP/1.1" useBodyEncodingForURI="true" redirectPort="8443" acceptCount="100" disableUploadTimeout="true"/>
  

• Increase the value of the maxHttpHeaderSize parameter e.g. maxHttpHeaderSize="32656" (If your connector tag doesn't have this parameter you should add it manually)
• Restart JIRA for the changes to take effect.

Workaround #2
Assign groups to project roles, instead of separate users