Unfortunately, the error I’m getting when I try to parse that CSR is not particularly useful (“asn1: syntax error: sequence truncated”, use this to reproduce).
My best guess would be that it’s due to the “C=Deutschland” field in your subject; the country field should be a two-letter string (DE for Germany). In fact, trying to generate a CSR with that subject fails with my version of openssl (with “140735108980816:error:0D07A097:asn1 encoding routines:ASN1_mbstring_ncopy:string too long:a_mbstr.c:158:maxsize=2”), though it will happily parse the CSR. I suspect the code used by Let’s Encrypt might be stricter here and that’s the issue.
If the software you’re using to generate the CSR allows you to get rid of all subject fields other than Common Name (CN), I’d recommend you do that, as Let’s Encrypt would remove those from the signed certificate anyway. If that’s not possible, “DE” should be an acceptable value.