Thanks to both of you!
Everything is ok!
My site is currently on a localhost. I've installed the OSC MS1 and the Spider Killer made by Henri Schmidhuber (available here - http://www.oscommerce.com/community/contri...ributions,1089).
My mistake was that while I was testing the spider killer I banned the IP of my localhost 127.0.0.1 and forgot to remove it from the list of banned IPs. So, then I had a problem with login. After deleting my localhost from the list everything goes ok.
IMHO, the Spider Killer is better than SID Killer made by Ian.
Thanks again for your fast response!!!