Hi
I upgraded to JIRA 7.1.8 from 6.4.12 in our test environment
Since upgrading, whenever I transition an issue where there is a validator in place, JIRA will hang while transitioning the issue with the progress wheel turning and eventually time out with the error in the transition screen:
“The call to the JIRA server did not complete within the timeout period. We are unsure of the result of this operation. Close this dialog and press refresh in your browser”
I can then no longer access the particular issue, the browser just sits there loading, however I can access all other issues in the JIRA instance and all other elements, dashboards, admin screens etc. In order to view the original issue I have to restart my JIRA instance
The presence of the following validators exhibit this behaviour
Field is required Validator - (JIRA Misc Workflow Extensions)
Fields Required - (JIRA Suite Utilities)
Field has single value Validator - (JIRA Misc Workflow Extensions)
However, these validators work fine
Universal Validator (Workflow Enhancer for JIRA)
Regular Expression Check (JIRA Suite Utilities)
The transition will report when it has failed validation without any problems - for example leaving a field blank when it is required. But will exhibit the above behavior if it complies with the validation.
Throwing this out there to see if anyone else has seen this 
Thanks in advance for any suggestions
Dave