There was an issue reported for the Operations monitoring apps whose reloads were failing with " Error: Connector is not responding (System error: WAIT_TIMEOUT)".
The script log showed for example that the error message appeared when trying to connect to a specific QLogs table/view in the PostgreSQL database for example "public"."view_performance_engine".
In case your engine(s) had an extended logging level and the troubleshooting step in point 4) solved the issue, then please be aware that according to R&D with the amount of data being pumped in and out.
Please try the following troubleshooting steps when you are facing this kind of error:
1) Try to increase the time out for the PostgreSQL connection as described in here: https://support.qlik.com/articles/000057138
2) Verify the database access as described in here: https://support.qlik.com/articles/000059076
3) Verify if the file data source is working. This can be done in the load script of each app in the section –by changing x from
SET db_v_file_override = 0;
SET db_v_file_override = 1;
4) Check the logging levels of the engine and set it back to the original state as seen in here: