It's not a HoI or Paradox question so much as it is a YouTube question, so this isn't really the best place to ask. You'd probably want a forum of YouTube content creators, or perhaps lawyers (who will inevitably give you the advice that you need to hire a lawyer for your specific case, since that's always the correct answer for legal questions, rather than trusting the advice from random people like me on the Internet).
YouTube copyright strikes are filed by copyright owners. They generally have automated filters which scan soundtracks of posted videos, looking for content that's similar to their music. "Similar", rather than exact, is a matter of necessity, both because an exact match would be easy to defeat with some trivial, inaudible changes, and because derivative works like cover songs are also covered by copyright law, so just recording your own version of something doesn't make it free from the original creator. Laws are similar in most countries (there's a treaty called the Berne Convention that standardizes minimum requirements for them), but you'll have to check your own jurisdiction for details and differences. YouTube itself mostly tries to stay out of the arguments, and seems to lean toward the copyright holders, probably because less or delayed content is better for them than being the target of lawsuits.
I don't speak German, so I'd have a hard time researching this particular issue. The song itself is from 1915, so it might be public domain, or might not. Berne makes copyright extend for lifetime plus 50 years, so the song could be in that period. Particular arrangements of songs can also have their own copyright, lasting after that period.
I did find at least one complaint that ICE_CS is one of those companies that files YouTube copyright strikes early and often. It's easy and cheap to let bots file copyright strike notices even without human intervention, so some people just spray them around. The strike might simply expire after 30 days. Or you can take up the gauntlet and challenge the copyright strike to get ICE_CS to drop it sooner by filing your own counter-notification with YouTube.
Here's YouTube's basic intro to copyright strikes:
https://support.google.com/youtube/answer/2814000?hl=en-GB