1.5.3 to 2.1.1: Error: Premature end of script headers: /powweb/web/cgi-bin/php4

I'm trying to migrate my Gallery 1.5.3 to Gallery 2.1.1. The migration went well for a few folders, but there are a few albums that it keeps crashing on:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster[at]gallery2[dot]skst[dot]com and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

The log file says:
Premature end of script headers: /powweb/web/cgi-bin/php4

There is no orange progress bar. Main.php sits there after I press Import, and then goes to the crash page. (I do see the progress bar during other operations, such as rebuilding thumbnails.)

I've validated G1; I found and deleted two orphans (not related to this album); I've downloaded the picture it seems to be stopping on, and uploaded it to G2 just fine. This morning, it seemed to upload three more pictures before crashing. Again, I was able to download and upload the picture just fine.

I deactivated GD--no change. (Although it did crash on a different image.) I re-activated GD and deactivated NetPBM--still crashes. I deactivated all image toolkits and EXIF--and I was able to import two albums I'm having trouble with. I tried importing more, and it crashed again. (Plus, I need at least one toolkit so it can resize the images if they're over the maximum size.)

The memory limit is 32M, and I have nowhere near 4,000 images--probably a few hundred. I tried increasing it to 64M--still crashes.

php.ini: memory_limit = 64M

Maybe related: In the albums I can import, there seems to be spotty thumbnail generation; some are generated, some aren't. In one album that crashed during import, the image that apparently caused the problem says "No thumbnail" on it and was placed at the beginning of the album instead of in its proper place. (I can view the image itself.)

Maybe also related: I tried to delete about a dozen albums--so I can start the import again--and it crashes with the same "premature end of headers"
error.

I thought perhaps the pre-installer didn't unzip all of the files correctly, so I re-uploaded the upgrade/ folder and its contents. Still crashes.

Thank you for any help or pointers you can provide.

Gallery version = 2.1 core 1.1.0
PHP version = 4.4.2 cgi
Webserver = Apache/1.3.34 (Unix) FrontPage/5.0.2.2635 mod_ssl/2.8.25 OpenSSL/0.9.7d PowWeb/1.1
Database = mysql 4.1.15-log, lock.system=flock
Toolkits = ArchiveUpload, SquareThumb, Getid3, NetPBM, Thumbnail, Exif
Acceleration = none, none
Operating system = FreeBSD clust05-www03.powweb.com 4.11-RELEASE FreeBSD 4.11-RELEASE #0: Thu Ma i386
Default theme = matrix
Locale = en_US
Browser = Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; .NET CLR 1.1.4322; .NET CLR 1.0.3705; .NET CLR 2.0.50727)

I had already re-ran the pre-installer to extract 2.1.1 over it again. I guess that wasn't "from scratch" enough.

I started totally fresh--new MySQL db, new tarball, etc. I installed by de-tarring it myself (via PHP) rather than use the pre-installer. I ensured there weren't even any warnings in the install, so I changed my php.ini to turn off output_buffering. (Maybe that was the problem??) I only installed NetPBM--no GD or EXIF. (I'll install EXIF after I'm done importing.)

Bottom line: It seems to be working now. The progress bar comes up right away. The only thing is that it stops at about 98% and doesn't say "Continue." It does display warnings as it truncates my captions though, and all the images seem to have been imported.

Thank you very much!

I tried to delete a user (and assign their items) to another user. It progressed for a while and then crashed: "premature headers" again. I checked, and it seems to have assigned all items to the new user, but the user to be deleted is still there. I tried to delete the user again, G2 says the user has no items, and it deleted the user.