Diagnose cause of 403 and 404 errors in Admin

[Cerulean]

I often get errors in Admin after submitting some data - a 403 at first because I have indexing disabled, but if I enable directory indexing I get a 404.

Mostly this relates to using modules, and the URL in admin is "http://mydomain.com/admin/moduleinterface.php"
I know this isn't the modules board, but my question is really about debugging. Is there a way to find out more about what is causing these errors? I have tried turning debug on in config but that doesn't give me any information on the 403/404 screen. There are no errors in my PHP logs.

I have had these errors in many modules (FormBuilder, CGblog, Glossary) so I think it must be some general configuration error, maybe to do with my hosting? Mostly the errors happen after submitting content containing HTML tags - it tolerates a simple structure but nested structures cause the errors, so maybe some kind of parsing issue? I know this description is very vague.

I saw another post where it was suggested that having mod_security installed in Apache can cause problems. Do you think that could be it?

Any help would be very much appreciated.

[Dr.CSS]

If you think it has to do with your server you need to post your system config, which is a good idea especially in cases like this...

[Cerulean]

System info:

Cms Version: 1.10.3

Installed Modules:

CMSMailer: 2.0.2
CMSPrinting: 1.0
FileManager: 1.2.0
MenuManager: 1.7.7
MicroTiny: 1.1.1
ModuleManager: 1.5.3
News: 2.12.3
Search: 1.7
ThemeManager: 1.1.4
TinyMCE: 2.9.5
AdvancedContent: 0.9.2
GBFilePicker: 1.3.1
CGSimpleSmarty: 1.4.10
FormBuilder: 0.7.1
TemplateExternalizer: 2.0.6
CGExtensions: 1.27.7
Gallery: 1.5.3


Config Information:

php_memory_limit:
process_whole_template: false
output_compression: false
max_upload_size: 350000000
default_upload_permission: 664
url_rewriting: mod_rewrite
page_extension: /
query_var: page
image_manipulation_prog: GD
auto_alias_content: true
locale:
default_encoding: utf-8
admin_encoding: utf-8
set_names: true


Php Information:

phpversion: 5.2.17
md5_function: On (True)
gd_version: 2
tempnam_function: On (True)
magic_quotes_runtime: Off (False)
E_STRICT: 0
memory_limit: 256M
max_execution_time: 60
output_buffering: On
safe_mode: Off (False)
file_uploads: On (True)
post_max_size: 15M
upload_max_filesize: 350M
session_save_path: /tmp (1777)
session_use_cookies: On (True)
xml_function: On (True)


Server Information:

Server Api: cgi-fcgi
Server Db Type: MySQL (mysql)
Server Db Version: 5.5.22


Also, my host has a few PHP functions disabled: symlink, shell_exec, exec, proc_close, proc_open, popen, system, dl, passthru, escapeshellarg, escapeshellcmd
If any of these are specifically needed I can get them enabled, but they wont enable all of them "just in case" because they are concerned about security (it's shared hosting).

[Cerulean]

For anyone else having similar problems:

The 403/404 errors were caused by having mod_security installed on my hosting server. I asked the hosting company to disable mod_security on my accounts and all errors disappeared. What a relief!