Cannot receive Mail

Hello,
I am using thunderbird portable for years and it worked fine until yesterday.
I got a new PC, Dell Latitude 5570, Win7 Pro, and wanted to get my mail using the stick.
But neither receiving nor sending worked.
All mailboxes are pop3.
After changing the port and authentication for the smtp server I could at least send messages.
Receiving does not work though I tried different parameters. I can access the mailbox via webmail.
The provider says everything is ok with those mailboxes.
When I press the receive button, it rushes through the mailboxes an nothing happens.

Thunderbird is uptodate.
Antivius is GDATA.

What is wrong?
Please help.
Thanks

Heinz