The private key has been imported into my wallet, but after this i cannot send my coins anymore.
Also i cannot dumpprivkey and import into other wallet because that gives an error.
So basicly my coins are in my current wallet but somehow i cant send them, and i cannot import the private key into an other wallet anymore.
Is your wallet encrypted? If so, try
this:
launch your bitcoin client as usual and wait for it to load the blockchain and start up
click on 'help' in the menu bar (top right)
click on 'debug window'
select the 'console' tab
type: walletpassphrase 600
type: dumpprivkey [your public key here]
this will return the private key, you can copy it now; ensure you clear your clipboard/history afterwards
type: walletlock
Typing "walletpassphrase 600" doesn't work if your passphrase isn't "600". Try the above, but replace the red text with your actual passphrase.
If your wallet's not encrypted, you can skip the "walletpassphrase" and "walletlock" steps.
If that still doesn't work, I'd suggest a rescan of the blockchain (close bitcoin-qt and run "bitcoin-qt --rescan" on the command line, it'll take a while to start). Try rebooting your computer too, that can solve a lot of problems.
EDIT: Forgot the wallet timeout
