Thanks for the reply!
I spent the last hour scouring the database.
Am I right that the TVs are located in _site_tmplvar_contentvalues ?
I looked for the specific tmplvarid, and discovered they are all still there. Good call!
However, the problem seem to be that the values have been altered (no idea how or why).
The one that works looks like, as you probably know, this
{"gal_id":156,"gal_name":"akb1.jpg","gal_filename":"9/156.jpg","gal_description":null,"gal_mediatype":"image","gal_url":null,"gal_createdon":"2012-02-01 20:55:32","gal_createdby":1,"gal_active":1,"gal_duration":"","gal_streamer":null,"gal_watermark_pos":"tl","gal_image_width":170,"gal_image_height":170,"gal_image_type":2,"gal_src":"/modx/assets/components/gallery/files/9/156.jpg","gal_orig_width":1891,"gal_orig_height":1891,"gal_album":"9","gal_watermark-text":"","gal_watermark-text-position":"BL","gal_other":"","gal_rotate":0,"gal_sizer":"9","gal_cropCoords":"","gal_cropTop":0,"gal_cropRight":0,"gal_cropBottom":0,"gal_cropLeft":0}
While the rest, that suddenly stopped working, look like this (included the three different variants of altered values):
{"gal_id":87
{"gal_src":false}
{"gal_src":0
As you can see, the values have more or less been "stripped". Some of the have lost the closing "}", some have been changed to false src or 0 src.
So at some point, these changes have occurred. Although I am uncertain as to when, why and how.