So, the underlying issue was:

For some reason, during the upgrade process, I managed to fuck up pict-rs working properly with lemmy. Note that I updated from scratch, not with docker or something similar.

What happened was, that pict-rs simply wasn’t running and being started any more alongside lemmy

My solution, which seems a bit hacky and improvised, admittedly, was to add a pict-rs service to systemd with the following config:

[Unit]
Description=pict-rs service
After=network.target

[Service]
User=lemmy
ExecStart=/usr/bin/pict-rs run -a 127.0.0.1:8080 filesystem -p /opt/lemmy/pictrs/files sled -p /opt/lemmy/pictrs/sled-repo
#Environment=LEMMY_CONFIG_LOCATION=/opt/lemmy/lemmy-server/lemmy.hjson
#Environment=PICTRS_ADDR=127.0.0.1:8080
#Environment=RUST_LOG="info"
Restart=on-failure
WorkingDirectory=/opt/lemmy/pictrs

# Hardening
ProtectSystem=yes
PrivateTmp=true
MemoryDenyWriteExecute=true
NoNewPrivileges=true

[Install]
WantedBy=multi-user.target

So, basically a copy of the lemmy.service, with pict-rs being started. If soemone has better solutions, or can tell me why I may have broken some security or smth, feel free to speak up. If this ended up helping you, from the future, with a similar problem, I am happy to hear that!