Unfortunately, the Austrian mirror still is on top of the mirrorlist now (I already tried a German mirror) but the certificate still is invalid 
Trying another:
Packages (1) cuda-12.3.1-1
Total Download Size: 541,53 MiB
Total Installed Size: 4476,28 MiB
Net Upgrade Size: 23,17 MiB
:: Proceed with installation? [Y/n]
:: Retrieving packages...
--2023-12-15 18:21:54-- https://mirror.it4i.cz/manjaro/unstable/extra/x86_64/cuda-12.3.1-1-x86_64.pkg.tar.zst
Loaded CA certificate '/etc/ssl/certs/ca-certificates.crt'
Resolving mirror.it4i.cz (mirror.it4i.cz)... 2001:718:1007:48::1:60, 195.113.250.60
Connecting to mirror.it4i.cz (mirror.it4i.cz)|2001:718:1007:48::1:60|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 567836381 (542M) [application/x-tar]
Saving to: '/var/cache/pacman/pkg/cuda-12.3.1-1-x86_64.pkg.tar.zst.part'
/var/cache/pacman/pkg/cuda-12.3.1-1-x 100%[=======================================================================>] 541,53M 29,0MB/s in 22s
2023-12-15 18:22:16 (24,3 MB/s) - '/var/cache/pacman/pkg/cuda-12.3.1-1-x86_64.pkg.tar.zst.part' saved [567836381/567836381]
(1/1) checking keys in keyring [----------------------------------------------------] 100%
(1/1) checking package integrity [----------------------------------------------------] 100%
error: cuda: signature from "Sven-Hendrik Haase <sh@lutzhaase.com>" is invalid
:: File /var/cache/pacman/pkg/cuda-12.3.1-1-x86_64.pkg.tar.zst is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
Am I the only one affected?
Moderator edit: In the future, please use proper formatting: [HowTo] Post command output and file content as formatted text