ServiceException code="InvalidCRS" (qgis-server:3.0.3)

[hbruch]

After switching from kartoza/qgis-server:2.18 to kartoza/qgis-server:3.0.3 I encounter various issues. Besides #17, the GetMap requests now respond with error code 400:

<ServiceExceptionReport xmlns="http://www.opengis.net/ogc" version="1.3.0">
 <ServiceException code="InvalidCRS">Could not create output CRS</ServiceException>
</ServiceExceptionReport>

This might be caused by some path/config issue, as the log indicates some database access issues(?):

...
[84][12:42:10] Can't open database: unable to open database file
[84][12:42:10] Can't open database: unable to open database file
[84][12:42:10] Can't open database: unable to open database file
[84][12:42:11] Error, could not create output CRS from EPSG

[hbruch]

Closing as duplicate of #16