Previews and 'normal' picture view are very small

Nextcloud version (eg, 12.0.2): 14.0.6snap1
Operating system and version (eg, Ubuntu 17.04): Ubuntu 18.04
Apache or nginx version (eg, Apache 2.4.25): 2.4.29-1ubuntu4.5
PHP version (eg, 7.1): 7.2

The issue you are facing:
When opening photos in either the Gallery or Files view the photo is shown very small. In the past I had these settings:
nextcloud.occ config:system:set preview_max_x --value=512
nextcloud.occ config:system:set preview_max_y --value=512
nextcloud.occ config:system:set preview_max_scale_factor --value=1

I soon realised (after opening only a few photos) that these values were too small so reset it to the default:
nextcloud.occ config:system:set preview_max_x --value=null
nextcloud.occ config:system:set preview_max_y --value=null
nextcloud.occ config:system:set preview_max_scale_factor --value=10

The problem is now everything seems permantently stuck in 512x512

Is this the first time you’ve seen this error? (Y/N): N/A