Thanks for your report.
Though I do not think this is a problem of WinSCP. You should better contact 1&1 support.
Note that winscp.net is hosted by 1&1 too! Never seen this problem.
Let us know if you find anything.

PS.
I tried the same sftp/username/passwd with FireFTP from Firefox: No Problem.

1&1 is germans largest internet provider with more than 10 million domains.
Connect with winscp and sftp to their webspace.
Upload a random file into a random directory.
php will no longer work in no domain of your webspace.
Even if you had different php-versions in different domains running.

But no panic:
Just close your winscp and everything will be ok.

You can do the same thing again - php will stop working
with an Internal server error on all domains of your webspace!!
If you just overwrite an existing file it will be the same.
There is no need to do this with php-files - any file will do.

Again:
Just close your winscp and everything will be ok.