APM Webtop Error. VPN tile unclickable

BIG-IP 15.1.0.4 Build 0.0.6

We have a VPN setup using APM. Our users access VPN via a Webtop tile after authenticating to the Webtop. Occasionally, a user will not be able to click on the VPN tile. Other resources on the Webtop work fine though, SAML pages, RDP, Hosted Content downloads, etc. When looking at the Developer Tools on the client browser, we see the following error every time the VPN tile is clicked:

Uncaught ReferenceError: custom_protocol_handler.JS?v=1:136

ExpirableStatusCheck is not defined

at CustomProtocolHandler.Run (custom_protocol_handler.js?v=1:136)

at NACPStart (webtop.eui?webtop=/Common/WT_Name_of_Webtop&webtop_type=webtop_full:358)

at Object.<anonymous> (webtop.eui?webtop=/Common/WT_Name_of_Webtop&webtop_type=webtop_full:601)

at APMSessionTimeout.rqStatusChange (session_check.js?v=13:115)

at XMLHttpRequest.<anonymous> (session_check.js?v=13:86)

This issue has occurred several times over the last couple of months, with no obvious trigger. In the past we have been able to solve it by duplicating the policy and reassigning it or restarting the F5 completely. This is not ideal though since it kicks out all our users.

Has anyone experienced this error before? Or can guide me in the right direction?