Error relocating /bin/opkg: getopt_long_only: symbol not found

I'm really lost with this one. I have a Turris Omnia 2020 that I just installed openwrt on by following these instructions. The installation went fine, but when I tried to install anything with opkg (or just run opkg) I got this error message:

Error relocating /bin/opkg: getopt_long_only: symbol not found

I think the symbol is from the getopt.h from libc, but (unsurprisingly) I have /lib/libc.so.

I downgraded to 19.07.8 in the hope that might help, but the problem persisted. Any ideas what I should try next?