That's no doubt a software issue, reported across forums after an upgrade to ICS 4.0.x.
Some users reporting this can be found here:
Don't know with certainty if you've upgrade the firmware to ICS 4.0 or any other variant, being the latest ones 4.0.3 or 4.0.4
Nonetheless, you either role back and wait for a new version that doesn't cause that issue or you can try some on the suggestions provided below (all reported to have worked without problems by other owners of a Samsung Galaxy S2).
Extending the answer:
First and above all
Backup all personal data on your phone to make sure you are not going to loose anything either by rolling back or trying a new/different firmware.
e.g., contacts, SMS, MMS, Internet settings, Wi-Fi passwords, etc...
You can go with one of this suggestions:
Once the backup is concluded, the following solutions are documented as to have solved that particular issue on your phone. Solutions ordered from the simplest to the most complicated:
Clearing cache data
The problem you're facing can be related to old files still in cache. Follow this steps to clear your cache and see if it solves it:
Boot your phone in recovery mode, to do that press and hold VOLUME UP and MIDDLE KEY, while holding both press POWER ON;
You will see your phone booted in recovery mode , in recovery mode touch screen will not work so use Volume UP and Volume Down to navigate through options and MIDDLE KEY to confirm;
Choose “wipe cache partition” from menu;
It will clear the cache data;
Now select reboot system now;
Samsung has a master reset code used to factory reset your phone. You can try this as to cleanup your phone and wait for a better ICS version, or try another ICS version (see suggestion n.º 3):
Make sure you have a backup of your personal data (e.g., contacts, SMS, MMS, Internet settings, Wi-Fi passwords, etc).
Take your SIM and memory card out of your phone
Turn your phone back on, and wait for the dial pad
Dial the factory reset code:
This will wipe, reset and then correctly format the internal storage. After the process is completed, place your SIM and memory cards back on the phone.
Firmware Flash (Odin)
Download the Latest Samsung Odin here, and install either one of this lastest ICS versions or a previous one that can be found on the links at the very end of this answer.
3.1. flash your firmware to the Android 4.0:
3.2. Install Android ICS 4.0.3 XXLPJ Firmware Update:
3.3. flash your firmware to the Android 4.0.4:
Useful and related links: