FATAL ERROR: Couldn't agree a key exchange algorithm | How to remove known hosts of sourcetree?

Since start of this week, I have trouble connecting to the git repositories of my private servers.

When I try to push, pull or fetch, I receive the following error type

git -c diff.mnemonicprefix=false -c core.quotepath=false -c credential.helper=manager-st push -v --tags xxxx master:master
Pushing to xxxxx
FATAL ERROR: Couldn't agree a key exchange algorithm (available: curve25519-sha256@libssh.org)
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

With putty, I can log in via ssh without trouble. When i use the mingw32 console, I can execute the commands as well, although I need to type in my password everytime for the private key, since I haven't configured an ssh client for the console.

The problem stems from some change on side of the managed servers, I assume.

It's quite annoying, because I can't manage my remotes anymore in the client.

I think, it could be solved by deleting the known hosts of sourcetree. But how can I do that?