I tried a relative path as you suggested and got the same result. I do, however have some more information . . .
The error message was deceiving: the path in the error stopped at the first directory it couldn't find. I added directories to build the path it was looking for and finally came up with this:
script '/var/www/html/var/www/html/plugins/tfu/tfu_file.php' not found or unable to stat, referer:
https://*******/plugins/tfu/tfu_210.swf ... lugins/tfu
As I said before, I have $install_path defined as /var/www/html/plugins/tfu in tuf_login.php. For reference the path of the calling page is "/usr/etc/webnative/styles/upload/dptfu/upload.html" (it's a prepress asset management site). I have also tried a relative path as you suggested but got the same error. Both versions of the $install_path definition work because the other scripts in that directory are found and seem to function as expected.
Here's the kicker: If I put an obvious error in $install_path, such as changing "tfu" to "tfuss", the other scripts fail as expected, but the error for tfu_file.php still references "/var/www/html/var/www/html/plugins/tfu/tfu_file.php". The typo is not included.
Given this new information, any other ideas where I need to be digging?
Thanks,
T.