Sorry, it didn't work.
I know it is permission related. I've included www-data in the mythtv group and made sure the mythweb/data belongs to www-data:www-data. And "data" allows group writing.
The png files are supposed to be inside of the recordings directory along with the ts files. ts files are fine, no png is found.
I did not build from the source code, it is running on 2020-02-13-raspbian-buster, not the lite nor the full.
What could go wrong? I have attached the screenshot, but you all know what I am referring to.....
Just manually ran the mythpreviewgen:
mythtv-light-30<192.168.1.101>:/opt/disk01/recordings:/>sudo -u mythtv mythpreviewgen --size 0x0 --chanid 10201 --starttime 20200327211600 --outfile /tmp/a.png
2020-03-27 17:38:16.306915 C mythpreviewgen version: fixes/30 [v30.0-60-gb9c1b41a6c] www.mythtv.org
2020-03-27 17:38:16.307100 C Qt version: compile: 5.11.3, runtime: 5.11.3
2020-03-27 17:38:16.307112 N Enabled verbose msgs: general
2020-03-27 17:38:16.307179 N Setting Log Level to LOG_INFO
2020-03-27 17:38:16.318318 I Added logging to the console
2020-03-27 17:38:16.320795 I Setup Interrupt handler
2020-03-27 17:38:16.320835 I Setup Terminated handler
2020-03-27 17:38:16.320858 I Setup Segmentation fault handler
2020-03-27 17:38:16.320878 I Setup Aborted handler
2020-03-27 17:38:16.320927 I Setup Bus error handler
2020-03-27 17:38:16.320950 I Setup Floating point exception handler
2020-03-27 17:38:16.320977 I Setup Illegal instruction handler
2020-03-27 17:38:16.321002 I Setup Real-time signal 0 handler
2020-03-27 17:38:16.321043 I Setup Hangup handler
2020-03-27 17:38:16.321847 N Using runtime prefix = /usr
2020-03-27 17:38:16.321867 N Using configuration directory = /home/mythtv/.mythtv
2020-03-27 17:38:16.322211 I Assumed character encoding: en_US.UTF-8
2020-03-27 17:38:16.325054 I Empty LocalHostName. This is typical.
2020-03-27 17:38:16.325123 I Using a profile name of: 'mythtv-light-30' (Usually the same as this host's name.)
2020-03-27 17:38:16.325584 I Start up testing connections. DB localhost, BE , attempt 0, status dbAwake, Delay: 2000
2020-03-27 17:38:17.438876 N Setting QT default locale to en_US
2020-03-27 17:38:17.438936 I Current locale en_US
2020-03-27 17:38:17.439146 N Reading locale defaults from /usr/share/mythtv//locales/en_us.xml
2020-03-27 17:38:17.474174 I Preview at calculated offset (240 seconds)
* failed to open vchiq instance
2020-03-27 17:38:17.629757 I AFD: codec AC3 has 6 channels
QWaitCondition::wait(): mutex lock failure: Invalid argument
QWaitCondition::wait(): mutex unlock failure: Invalid argument
QWaitCondition::wait(): cv wait failure: Invalid argument