Error when assigning modules

I get an error when I try to assign modules to a property.

Fatal error: Declaration of CustompropertiesTableCpvalue::delete() must be compatible with that of JTableInterface::delete() in public_html/administrator/components/com_customproperties/tables/cpvalue.php on line 136

I get this even if it´s an active module. I have noted that when you have deleted a module it will still show up in the Assigned modules field. A bug and the cause of the problem?

I use Joomla 3.3. I have updated Custom Properties to 3.1.5 with no errors, but still the About-page says "Version 3.1.3". Another bug?

The error you are reporting can only by caused by Custom Properties 3.1.3.
Since you still see 'Version 3.1.3' in the about page, I'd say the update procedure has not been successful and you're still running the old version of Custom Properties.

So what should I do? I have updated Custom Properties twice, and as I said, no errors. Should I try to not use the whole package but install component, plugin and so on?

Check the dates of the updated com_customproperties files on the server, to see if those files are actually being copied.

You can also try to update only the component, instead of using the installer.