maybe a stupid question. But how to log out from the webwallet?
THX
You don't have to log out from it as you are not logged in to a server but into a local cookie stored in your browser
what if I close my browser and delete the cookie or wanna run the webwallet on another pc?
Do I have to create the wallet again with the seed and will it show my transaction history, etc. ?
yes
ok, thank you
but is there an option to backup the wallet respectively the cookie?
AFAIK you can't backup the cookie but the seed is your wallet backup.
Decred web wallet is a hybrid one between an offline wallet and a regular web wallet(with username pass etc stored in server). The seed acts pretty much like the wallet.dat file in bitcoin-like clients.
If you are having the seed you can access your account from anywhere
thank you for clarification.
but informations like my chosen walletname will be lost or are they stored on the blockchain?
(not really important but I like to know that)
Edit: raetsch's post above answered everything

Thank you