Cannot redeclare class base in /includes/classes/class.base.php on line 17

I get this error when I try to see my admin page, so I cannot log in. Page where this is displayed:
http://www.c o m a n d a t i.ro/admin/login.php (with no blank spaces in the domain name)
I have recently upgraded from 1.3.7 to 1.3.8a with a one-click install from Dreamhost, and I applied the security patch plus a few changes (extended layout, ...). Yesterday, the admin login page was working ok, now it's not, I must have done something wrong, but I don't know what. I read all related threads on this forum and I still don't know what to do. I tried with WinMerge and I can't really compare the files.

Can I edit somehow class.base.php in order to not redeclare class base, or what can I do?

Thanks.

I guess one of the problems with knowing a bit about Zen Cart, is knowing that a one click upgrade is simply too good to be true for all but the most straightforward of sites, and any host promising that is being rather irresponsible.

I would strongly recommend against editing the class.base.php to inhibit the legitimate declaration of the base class. You really need to find the one that shouldn't be there that is getting in first.

Two ways to do this, firstly look for any back up files hanging around in the classes folder and get rid of them. If that does sort it out, use the developers tool kit in the Admin > Tools menu to find the other place that the class is being declared.

class.base.php is below, how can I edit it? I tried deleting one file at a time in the classes folder in /includes but to no avail, all files are back again now, so I have the same problem. Please help.

It has nothing to do with class.base.php. You can't edit that file to fix the problem, because it is not the problem.
You've likely got something else that has the same content, or is requesting that class.base.php be loaded a second time.

There is no simple answer other than the following:
If it were my site that was displaying that problem, I'd be firing up WinMerge and doing a complete comparison of every file against the original set of Zen Cart files for the version you're currently using. Identify "extra" files and "different" files, and sort out any differences.