Hi Markus
did the last job generate a job_log?
You are getting the error "parameter is incorrect" during the Connecting to system phase.. so (just thinking aloud.. 
)
- error in checking some permission? - is the user domain admin..
- error in checknng the password? - does the user password contains some special char? is the OS with the locale settings in English?
another thing you can do is to install the plugin manually. i mean via command line avoinding the use of the snapcenter GUI
look at this https://library.netapp.com/ecm/ecm_download_file/ECMLP2838332 pag 35
Do not use the installer GUI to install SnapCenter Plug-in for VMware vSphere on the SnapCenter Server host