Steps to reproduce:

1. Configured the zabbix web service, made sure it can see the zabbix server.
2. Navigate to screen Reports, Sheduled reports. Fill in the dashboard, start end time etc. 
3. Use the Test button or schedule the report for a specified time
4. The pdf arrives but the graphs are empty
5. There are no error logs in the zabbix web service logs (log level set to 4)

Result:
See screenshot...

Nothing meaningful in the log file:

2021/11/12 08:40:00.259937 received report request from XXX.XX.X.X:XXX
2021/11/12 08:40:02.623979 writing response for report request from XXX.XX.X.X:XXX

Expected:
Expected results is that report will contain graph if not that the zabbix web service logs why the graph generation failed. Almost no errors are logged at this point.