No dashboard after upgrading to 2.5

When I visit https://www.maha-online.de/blog/wp-admin/upgrade.php what I see is an ‘old’ upgrade screen telling me you may not have uploaded all the 2.5 files correct.

Please review steps 7 and 8 of Upgrading WordPress Extended.

i have the problem
after login was found the message:
Fatal error: Call to undefined function: hash_hmac() in /home/i—–/public_html/optikonline/wp-includes/pluggable.php on line 518

help me

Now everthing is OK

Thnaks

MichaelH: that is because I have restored the old version. I followed steps 7 and 8 to the letter, as far as I can see.

I have retried to upgrade, following exactly the stepts in the guidelines. The same result! I have downgraded once again. I got to writing the following line into my browser’s address field:

https://www.maha-online.de/blog/wp-admin/upgrade.php

It produced a white page. Then I typed in:

https://www.maha-online.de/blog/wp-admin/

and got another white page while the address line changed to:

https://www.maha-online.de/blog/wp-admin/upgrade.php?_wp_http_referer=%2Fblog%2Fwp-admin%2F

(Just one line)

What does that mean? Please, give me at least a hint where I can look for a solution. I never had any problems with upgrading so far, and I did not deviate from the instructions, so there must be something wrong here.