If you suspect Suhosin may be posing a problem for you, the easiest way to check is to disable it.
From Admin > Server > PHP Info, note the "Path to PHP.ini" listed. Edit this file in a text editor and find the Suhosin configuration section marked by [suhosin] and add a new line:
Save the php.ini file and then restart your web server. Back on Admin > Server > PHP Info you should see suhosin.simulation listed as "On".
If you find Suhosin was indeed the problem you may want to keep it disabled. Alternatively, if you want the extra security Suhosin provides, you can enable the log (see http://www.hardened-php.net/suhosin/configuration.html#logging_configuration) to see what is actually happening and then tweak the configuration responsible.