installing a package from private gitlab server on windows

I am struggling with installing a package from a gitlab repository on a windows computer.

I found different hints but still have problems to install my package from gitlab. First of all, I generated a public and private key with puttygen.exe
The files need to be changed afterwards, I had to remove comments and stuff so they look like my the file on my unix system. So now, both public and private key files have just a single line.

I tried to install my package via devtools::install_git which takes very long and I get the error message

Error: Failed to install 'unknown package' from Git:
  Error in 'git2r_remote_ls': Failed to authenticate SSH session: Unable to send userauth-publickey request

And with devtools::install_gitlab I get a different error message and I somehow have the feeling, the link which gets generated doesn't fit to my gitlab server.

Error: Failed to install 'unknown package' from GitLab:
  cannot open URL 'https://gitlab.rlp.net/api/v4/projects/madejung%2FMQqueue.git/repository/files/DESCRIPTION/raw?ref=master'

My complete code to test at the moment is

creds <- git2r::cred_ssh_key(publickey="~/.ssh/id_rsa_gitlab.pub",
                             privatekey="~/.ssh/id_rsa_gitlab")
devtools::install_git(
  url='git@gitlab.rlp.net:madejung/MQqueue.git',
  quiet=FALSE, 
  credentials=creds)

devtools::install_gitlab(
  repo='madejung/MQqueue.git',
  host='gitlab.rlp.net',
  quiet=FALSE, 
  credentials=creds
)

My id_rsa_gitlab.pub file looks like this and is just a single line

ssh-rsa AAAA....fiwbw== rsa-key-20200121

The id_rsa_gitlab file has just the code

AAABA.....3WNSIAGE=

UPDATE

On my Mac system it works as expected after installing the libssh2 library via homebrew and and recompiling git2r with install.packages("git2r", type = "source").

So the working code on my machine is

creds <- git2r::cred_ssh_key(publickey="~/.ssh/id_rsa_gitlab.rlp.net.pub",
                             privatekey="~/.ssh/id_rsa_gitlab.rlp.net")
devtools::install_git(
  url='git@gitlab.rlp.net:madejung/MQqueue.git',
  quiet=FALSE, 
  credentials=creds
)

For some strange reason, the devtools::install_git call needs about a minute to fail in the end. I have no idea where the problem here is.