Direct link rendered image 404 error, log useless


I am using grafana 5.1.4 on raspberry pi under raspbian (openhabian).
When I click “direct link rendered image” I only get an 404 error…

The log shows nothing useful…

t=2018-10-08T23:10:18+0200 lvl=info msg=Rendering logger=png-renderer path="d-solo/000000002/test?refresh=1m&orgId=1&from=now-1h&to=now&panelId=1&width=1000&height=500"
t=2018-10-08T23:10:22+0200 lvl=dbug msg="[phantom] Loading a web page: http://localhost:3000/d-solo/000000002/test?refresh=1m&orgId=1&from=now-1h&to=now&panelId=1&width=1000&height=500&render=1 status: success 60000" logger=png-renderer
t=2018-10-08T23:10:23+0200 lvl=dbug msg="Image rendered" logger=png-renderer path=/var/lib/grafana/png/69dmxetq7Wc3otb2S2fv.png
t=2018-10-08T23:10:23+0200 lvl=info msg="Request Completed" logger=context userId=1 orgId=1 uname=admin method=GET path=/render/d-solo/000000002/test status=404 remote_addr= time_ms=4433 size=19 referer=

There is no image at /var/lib/grafana/png

What can I do now?
I have seen many github issues with almost the same problem but there never was a solution…

Thanks and best regards!

Solved by backing up grafana.db and grafana.ini, then reinstalling grafana and restoring the files.