Truncated URL's caused by .htaccess?

My blog is hosted in a subdirectory of my main site (…/blog) and both the root directory and subdirectory have .htaccess files which, as far as I am aware, have not been edited by either myself or the web designers.

The problem I am having is best described by the designers of my blog software, ProPhotoBlog:

“It appears that your web server is truncating the URI of that “Upload Image” button. The button should be attempting to access this URI:
http://karlbungey.co.uk/blog/wp-content/themes/prophoto3/adminpages/popup.php?do=upload&p3_image=masthead_image3&misc=false&TB_iframe=true&width=410&height=110
but it appears to actually be calling:
http://karlbungey.co.uk/blog/wp-content/themes/prophoto3/adminpages/popup.php?do=upload&p3_image=masthead_image3&misc=false&

As you can see, much is missing.
Can you contact your web support folks at your hosting company to verify your server configurations? I see several special rules created in the “.htaccess” server configuration file in your “public_html” directory which may be the culprit. Be sure to mention the issue I state above – they should be able to get you squared away quickly.”

I have contacted my host, HostPapa, and drawn a blank (also drew a blank on another issue I had with a previous blog template, so I suspect that they may not know what they are talking about! And have said that they are not supporting WordPress problems…

Does anyone understand this more than I do? I cannot find any reference to this issue in the forum or FAQ’s, but did find lots on truncating weblinks!