5
Was it a fresh installation of XOOPS?
In testing, I have only encountered in upgrade from XOOPS 2.3.3, but it was because I didn't delete the /modules/system folder. Once I did it, and copied the new /modules/system folder of 2.5.6 (which is recommended in our instructions), there was no issue.
Tell us more about your current XOOPS 2.5.5 installation, and what PHP version do you have.
How could I replicate it? Did anybody else encountered this issue?