Blank thumbnails with g2image and phpBB

I've read several of the posts about blank thumbnails in g2image but they don't seem to describe my situation. A few details:

I have two full installations of phpBB, Gallery and g2image. One is in a /public_html directory, and a second is in a sub-directory (/public_html/test) that is password protected and is my testing/development environment. Each installation has completely separate databases, etc..

The production system seems to work perfectly. However, the test system will always display blank images in g2Image chooser UNLESS I visit the gallery itself first. In other words, if I go directly to phpBB and use g2Image to try to attach a photo to a posting, the dialog appears blank. If I visit the gallery directly, and then come to attach a photo, the image chooser shows the thumbnails correctly. There appears to be some kind of strange session behavior.

The gallery itself in both cases works properly. I've looked through the configuration settings and they appear to be correct for both installations.

I actually have observed this behavior in two different sites I manage, both of which are structured in a similar way with a /test installation and a production installation.

Any suggestions where to look for this obscure problem?

Thanks!
Mark