Additional in the Apache error log file the following is listed:
[Sun Mar 26 23:33:47.129114 2017] [proxy_http:error] [pid 4034] (20014)Internal error (specific information not available): [client xx.xx.xx.xx:64572] AH01102: error reading status line from remote server 127.0.0.1:20000, referer: https://DOMAIN/SOGo/so/USER/Mail/view
[Sun Mar 26 23:33:47.129142 2017] [proxy:error] [pid 4034] [client xx.xx.xx.xx:64572] AH00898: Error reading from remote server returned by /SOGo/so/USER/Mail/0/view, referer: https://DOMAIN/SOGo/so/USER/Mail/view