theme: i meant whether you are using xbootstap, xswatch or what else.
because if it is the same with another theme we would know whether the problem is located in the theme files or not ;)
as I also cant reproduce your error I think that there is a problem with your XOOPS installation, maybe is one of the necessary files or folders broken or not readable. maybe you should overwrite your XOOPS with the actual version fromhttps://github.com/XOOPS/XoopsCore25