Embedded G2: blank pages with Gallery Block enabled

I have Drupal 4.7.0 and Gallery 2.1, both working well stand-alone. I have configured both as per the instructions provided in the Gallery module. I have enabled the Gallery module and have checked the paths, etc. and all are valid. The results of the configuration test are:

    * Embedded Gallery2 Status:
          o 'URI of Gallery2' variable: Success
          o PHP Memory Limit: Success
          o Gallery2 Init: Success
          o Gallery2 Image Block Module Status: Success
          o Gallery2 Image Frame Module Status: Success
          o Gallery2 URL Rewrite Module Status: Success
          o Gallery2 URL Rewrite Module Auto-Configuration: Success
          o Overall Status: Success
    * The configuration options have been saved.

Whenever I attempt to address any gallery pages through Drupal, enable either the Gallery Block or Grid Block, or if I select any of the Gallery2 Site Admin links, I get a blank browser screen - I normally have to disable the block manually in the database (via phpMyAdmin and the blocks or system tables) in order to get the "blocks" Admin page to display anything. I can sometimes see Drupal pages (unless I've enabled a Gallery-related block, in which case all pages are blank, so it's obviously related to Gallery). Investigation of the relevant Apache/PHP logs sometimes (but not always) shows a segmentation fault, e.g. [notice] child pid 23821 exit signal Segmentation fault (11). Oddly enough, I can enable the Gallery Navigation block without getting a blank page, but it doesn't display anything.

Has anyone else encountered this?? I'm guessing it might be a PHP-related problem, but I haven't seen anything else on either the Drupal or Gallery sites, nor anything relevant in Google. Any insight would be much appreciated.

For the record, my setup:
Drupal 4.7.0 (upgraded from 4.6.6) - theme: bluemarine (same problem with my custom theme), no non-default modules except Gallery
Gallery 2.1.0 (upgraded from 2.0.4)
Gallery Module 4.7.0 (and also tried CVS, but it is identical to the current 4.7.0)
Server Platform: Ubuntu Linux (Dapper beta), PHP 5.1.2-1ubuntu2, Apache 2.0.55-44, MySQL 5.0.19-3
PHP Memory available: 32MB RAM

Thanks in advance,

Dave