This could be caused by flashing an incompatible GApps package. If that's not the case (and you are sure you have the correct/compatible GApps installed), try the following:
- Open the "Google Play" app and press the home button to return
- Go to Settings → Applications → Manage Applications
- Select the "ALL" tab, Search for "Google Play Store" and press it to open.
- Press "Clear cache"
- Press "Force stop"
- Return to Settings → Applications → Manage Applications
- Press "Google Services Framework"
- Press "Clear Data"
- Press "Force Stop"
- Start "Google Play Store" and it should give an error
- Reboot. Once your device has restarted Google Play Store should work. Wait a minute or too if it doesn't.
This should help if a corrupt cache is the cause of your problems (a corrupt cache might lead to strange error messages, amongst others). If it doesn't help, you might try the same steps but include "Clear Data" for the playstore app.