Error - No thumbnail-resizing in the MODX Filemanager (Media-Source: Gallery) since MODX 2.2.7 & 2.2.8

Hey Guys,
I've got a big problem with the MODX Filemanager in combination with the Gallery Media Source (Gallery-Version: 1.5.2).
Since I use MODX 2.2.7 / 2.2.8 for newer projects the MODX Browser/Filemanager is unable to resize the images in the gallery-albums for selecting. (screenshot attached)
I've looked at older Projects (MODX 2.2.4 and MODX 2.2.6 - both also with Gallery 1.5.2, with the same Media-Source-Options and on the same server), there is no problem. The images get resized to the viewport, the thumbnails should have for previewing.

Has anybody an idea how to fix that quickly?

Many thanks and greetings from Germany
Jens