Error when installing module PageDelete

[Seb 1]

Hello,

I tried to install nico's module Page Delete ( http://modules.processwire.com/modules/process-page-delete/ ) and get the following error:

Unable to install module 'ProcessPageDelete': Template 'admin' has the system flag and you may not change it's 'id' or 'name' fields.

I've no idea what this means.

I use the useful module with some other ProcessWire installations on the same server and never had this problem.

Can you help me?

Thanks in advance