first something I forgot: try to enable PHP Debug in XOOPS admin -> system -> preferences -> general -> debug mode and see if there are some clues. There will be many Notices, but that is normal, look for Warnings and errors.
Now, about the log files - the web server logs some events in files. It depends on the server and settings where they are stored. It could be several of them - for access, for errors ... If you use services of hosting company, you may not have access to those files and need to contact the support. An apache error log looks something like:
PHP Warning: Unknown(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php4) in Unknown on line 0
PHP Warning: set_time_limit(): Cannot set time limit in safe mode in /var/www/html/db.php on line 23
PHP Warning: session_start(): open(/var/lib/php4/sess_d54b81024cf2, O_RDWR) failed: No such file or directory (2) in /var/www/html/db.php on line 25
PHP Warning: Unknown(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php4) in Unknown on line 0
[Mon May 29 20:20:56 2006] [error] [client 192.168.1.2] (13)Permission denied: file permissions deny server access: /var/www/html/modules/rmms/uploads/logo_6E_feLCC.gif
modsec is apache module that provide aditional security for apache server. It may have separate log file. However it usually it gives 403 Forbiden error if it block something, so probably that is not the problem.
most modules have a settong about allowed file types, size and so on as well as the upload folder for that module.
php.ini is file that have settings for PHP. It may not be accessible for you too. it has this settings that may change/limit uploads:
;;;;;;;;;;;;;;;;
; File Uploads ;
;;;;;;;;;;;;;;;;
; Whether to allow HTTP file uploads.
file_uploads = On
; Temporary directory for HTTP uploaded files (will use system default if not
; specified).
upload_tmp_dir = /??????/??????/
; Maximum allowed size for uploaded files.
upload_max_filesize = ????????M
could have more depending on server/settings.
If you use hosting service try to contact the support, explain the problem and they may be able to track it.
If you use test sistem on your computer (XAMP for example) you will have access to all settings/log files
