Prints getting worse, need help fixing this  

Prints getting worse, need help fixing this

Hi all,

I received my mini in june an at first it printed like a charm. I also have an i3mk3s and the prints from both were equally great, hardly any layer lines, and so on.

About a month ago the prints from the mini started showing some stringing, which got gradually worse to an extremely annoying point where prints will be useless due to the webbing which is hard to remove. About a week ago I also started getting under extrusion issues, but not very consistently.

Over the past weeks I have tried temperatures ranging from 185C to 210C (normally I print my pla at 200C), lower and higher speeds, new spool of filament, retraction settings, cold pulls, hot pulls and checked the tube for smooth filament-gliding. I have no idea what settings I specifically tried, probably all of them.
Tonight I reverted back to basic default settings and that changed absolutely nothing.
I did not mod or alter anything technical, and I did not spray of put anything in the tubes.
I have lubed the metal moving bits sparingly when assembling.

There are occasionally clicking-sounds either in the hot-end or the other part where the filament runs through, but not often. I did not have those at first, this is also something from the past weeks.

I did notice last week that there is black stuff on the lower x-axis metal rod, like sooth or something. I am not sure what could cause this, and if it might be related.

I print with IOP pla at 200C, I tried altering spools that gave no issues on the i3mk3s to the mini, got the stringing+underextrusion, changed them back, prints were fine. I have only printed with pla on my mini.

Does anyone have any ideas what I might try to fix this?
I am getting rather frustrated with the mini, especially since I had no issues at all the first 2 months, and I print almost 24/7.

Thank you very much in advance for any help!