But unfortunately nothing happens. I configured the Location to be "file" and the directory to be "Persistant Data Path", but I don't see the file time.es3 created in that directory. There also is no error message, it just doesn't seem to do anything.
You have the parameters wrong. The first parameter is the key, the second parameter the value you’re saving (or the defaultValue in the case of ES3.Load), and the third parameter the filename.
I'm sorry but after reading in the API I don't really get why my code shouldn't work. Here it states that the filePath is optional. So where is the data saved when I omit the file path?
should store the float returned by GetActualTime under the key "time.es3" (I just changed it to just "time" but it won't work either). Or did I misunderstood something? Without a file path is the data saved in the same file as the autosave values?
Your code should technically work, but it won't create a file named time.es3. It will save it as a key named time.es3 in the default save file (usually SaveFile.es3) in the persistent data path (which you can open by going to Tools > Easy Save 3 > Open Persistent Data Path).
If this isn't happening for you, please could you create a new script which I can drop into a new scene of a new project which replicates your issue so I can see what is happening, as I've had no other reports of this and it appears to work fine for me.