You are using an older version of SMF with errors and problems. As soon as possible is recommended that you update the version of your forum en son.
I do not know much English, I'd appreciate if you help.
You will need to download and upgrade your forum from SMF 1.1.x to SMF 2.0 Gold which is more stable than SMF 1.1.x. When you go to this page (http://download.simplemachines.org/), you will need the Large Upgrade package, unzip it, upload the extracted files to your SMF directory, and then navigate your browser to http://www.yoursi.te/smf/upgrade.php. Before doing so, make sure to backup the necessary files before uploading the files from the large upgrade package.
2.0 is NOT more stable then 1.1.1*. 2.0 has MORE features then 1.1.1*.
they should be equally stable
It is true though that there have been numerous security fixes in 2.0, so that can count as some kind of stability security wise right? :)
any security fixes in 2.0 would either be fixed in the 1.1.1* branch or issues that are not found in the 1.1.1* branch so the fixes do not apply.
both 1.1.1* and 2.0 should be equally stable and secure