The upload bug seems to have solved itself. Not sure what was causing it, but all seems well now.
The change in the opening php tags should be added here in the WordPress Plugin Repository soon, so others won’t run into it.
Thanks Malek for the support (provided by e-mail and on his personal site).
These bugs were fixed by changing the opening php-tags (<?) to (<?php) in the following files:
* wm_functions.php
* wm_dir.php
This took care of the error messages I posted above, but uploading a file now results in an error. Watermarks are not being applied, and the error message contains no information as to what’s causing it. It might be a conflict with another plugin, not sure. I’ve contacted the author of this plugin and hope to resolve it soon.