You may notice fiber loop errors appearing in the Windows, QlikView and Qlik Sense logs.
StdServer: Fiber loop (10b74b020) stall detected: last fiber loop iteration started 120917 ms ago.
Fiber loop is an entirely normal error to see and is usually just a sign of a busy server. It may not show any symptoms to the end user. Fiber-loop stalls indicate a problem with a Windows fiber (a kind of thread) that has taken too long to respond. It shows up in the Windows error logs and the user may experience this a client crash as the session has been closed. If the browser is refreshed then a new session is created.
If the peak memory usage is sufficient to drive the server up towards the upper control limit (High Working Set default is 90%), then one can expect to see some level of degraded performance and unusual failures as the in-engine cache will be mostly drained of memory. The required and recurring re-calculations of results will also use a lot of CPU, and this is also likely the path of explanation for the observed stalls.
QlikView 12.00, 12.10 and 12.20 (November 2017)
Qlik Sense 3.x , June 2017, September 2017, November 2017, February 2018 and onwards
Fiber-loop stalls indicate a problem with a Windows fiber (a kind of thread) that has taken too long to respond. Generally caused by a busy server that may or may not be oversubscribed (causing peaks in resource usage) and/or by slow network performance particularly when working with large apps stored on network device or share.
It would be more a case of investigating the overall server performance, size of the Apps (if these are very large and stored on a network device) perhaps there are issues with Network speed/performance. Any performance issues should be investigated with their local IT team to help reduce the number of fiber loop errors reported.