I have a hosted webserver with http/2 (medium fast) and additionally I have a space on a fast CDN-Server with only http/1.1. Is it recommended to load some ressources from the CDN or should I use only the webserver because of http/2? Loading too many recources from the CDN could be a bottleneck due to http/1.1? Would be kind to get some hints...
You need to test. It really depends on your app, your users and your servers.
Under HTTP/1.1 you are limited to 6 connections to a domain. So hosting content on a separate domain (e.g. static.example.com) or loading from a CDN was a way to increase that limit beyond 6. These separate domains are also often cookie-less as they are on separate domains which is good for performance and security. And finally if loading jQuery from code.jquery.com then you might benefit from the user already having downloaded it for another site so save that download completely (though with the number of versions of libraries and CDNs the chance of having a commonly used library already downloaded and in the browser cache is questionable in my opinion).
However separate domains requires setting up a separate connection. Which means a DNS lookup, a TCP connection and usually an HTTPS handshake too. This all takes time and especially if downloading just one asset (e.g. jQuery) then those can often eat up any benefits from having the assets hosted on a separate site! This is in fact why browsers limit the connections to 6 - there was a diminishing rate of return in increasing it beyond that. I've questioned the value of sharded domains for a while because of this and people shouldn't just assume that they will be faster.
HTTP/2 aims to solve the need for separate domains (aka sharded domains) by removing the need for separate connections by allowing multiplexing, thereby effectively removing the limit of 6 "connections", but without the downsides of separate connections. They also allow HTTP header compression, reducing the performance downside to sending large cookies back and forth.
So in that sense I would recommended just serving everything from your local server. Not everyone will be on HTTP/2 of course but the support is incredible strong so most users should.
However, the other benefit of a CDN is that they are usually globally distributed. So a user on the other side of the world can connect to a local CDN server, rather than come all the way back to your server. This helps with connection time (as TCP handshake and HTTPS handshake is based on shorter distances) and content can also be cached there. Though if the CDN has to refer back to the origin server for a lot of content then there is still a lag (though the benefits for the TCP and HTTPS setup are still there).
So in that sense I would advise to use a CDN. However I would say put all the content through this CDN rather than just some of it as you are suggesting, but you are right HTTP/1.1 could limit the usefulness of that. That's weird those as most commercial CDNs support HTTP/2, and you also say you have a "CDN server" (rather than a network of servers - plural) so maybe you mean a static domain, rather than a true CDN?
Either way it all comes down to testing as, as stated at the beginning of this answer it really depends on your app, your users and your servers and there is no one true, definite answer here.
Hopefully that gives you some idea of the things to consider. If you want to know more, because Stack Overflow really isn't the place for some of this and this answer is already long enough, then I've just written a book which spends large parts discussing all this: https://www.manning.com/books/http2-in-action