Faltering playback and editing

I am in the editing process of my feature film, which has 118 scenes plus B-Roll, and each scene containing 4-20 clips.

I'm editing on a tower PC with a 4K Monitor, both able to handle the project.

I created the right sequence, not pulling the footage in the timeline, but creating a sequence that has the right settings according to my shot footage.

The clips have an aspect ratio of 3840 to 2160 with 50 fps. I then created proxies for all the footage, which got stopped just before it was finished with an unknown error popup. I had a look at if all the proxies are created and attached to each clip and that is the case. The clips have proxies attached.

I started editing and I am now around minute 5 in the timeline and the timeline, as well as the source/ preview window, where I decide which clip to use, are faltering. The sound continues, but the video freezes. 

The proxies are activated in both, the preview window as well as the program/ playback window through toggle proxies. 

The playback is on 1/4 in both windows.

If I render everything it runs smoothly until I insert one or two clips.

I don't know what I could have forgotten or overseen. 

Thank you for your help in advance!