No upload with files > 1KB

Hello Forum,
I use SmartFTP while a sicnce months, and now I have this problem:
I can't upload files that are bigger than 1 KB. SmartFTP sends the STOR command and then a few bytes and then - nothing. No more traffic, and then comes the timeout error.
I asked my provider. He said it's all correct with the server, it must be a local problem at me.
Can you help me? BTW, I use passive mode.
Thanks a lot!