I used the search function and did’nt find anything.
So, is it possible to monitor a server via vpn?
As an example: monitor the server @workplace from home.
If yes, how does it work? I tried and just can use the “ping” function. Snmp installed everywhere.
Answer would be nice, so thanks for may helping me 
If you are able to ping the device you can run a Dude server to monitor your server with SNMP produce graphs etc.
http://www.mikrotik.com/thedude.php
Install the dude server either on a Mikrotik device or using a local server that can ping the device (VPN or not is irrelevant as long as you can reach it). Log in with the client application and add the server as a device and add the services you want to monitor. Hit the red + on the network map and add the device.
If what you really want to monitor is the link, add the two devices by IP on either end of the VPN you want to monitor, and use the red plus to add a link, drag from one device to the other, choose to monitor via the end of the link you want to monitor and choose snmp type and appropriate link type (I choose point to point for VPN links they look like dotted lines on the map). Double click the link on the map and check the graph bit rate option in the history tab if you want it to keep graphs.
Hope that helps.