I'm new to the website and don't speak german, so forgive me for speaking English.
I have a problem with my fork, as one of the inner tubes seems to be 'sticking' or getting stuck when I put it together. I suspect the culprit is this part from the innertube

That little ring is somewhat stuck. I can move it only a little. The other fork doesn't have this problem. I suspect there's some dirt or whatnot stuck inside that prevents it from moving normally. I tried cleaning it with WD40 and an old toothbrush, but had no positive results. How can I fix something like this? Any suggestions or help are greatly appreciated!
Thank you for your help