[Koha] Error message at login: 'You do not have permission to access this page'

[Frank Peters]

Hi out there,

I'm glad to take part in this list in my effort to set up a working Koha 
database for the library of our institute.

Unfortunately, I encounter the first problem right at the beginning: 
When logging in with my database administrator account, I get this message:

"Warning: You're logged in with the database administrator account. This 
is a bad idea, and you are likely to encounter problems. You should 
create a patron to use when administering Koha and give it 
superlibrarian permissions."

So I did: I've created a new patron with my name (peters), choosing the 
category "SuperUser (SU)" (the only available option). When I try to log 
in as this patron, however, I get the message:

"Error: You do not have permission to access this page."

So, what's going wrong?!

Grateful for any helpful idea!

Frank