Meta: this isn’t really programming or development, and would probably be better on superuser or maybe security.SX, but this is issue is likely to become more common as OpenSSL 3.0 spreads and I wanted to get the answer out.
OpenSSL 3.0.x (and up) by default doesn’t support old/insecure algorithms, but until recently most software that creates PKCS12 (including OpenSSL 1.x.x) used such an algorithm for the certbag(s), namely a PKCS12-defined PBE using 40-bit RC2, usually abbreviated RC2-40 – and some still does at least sometimes, like the Windows 10 cert-export dialog by default. To check this do
openssl pkcs12 -in my.p12 -info -nokeys -nocerts
# in 3.x.x add -provider legacy -provider default or just -legacy
# to avoid prompt use -password or -passin, see man pages
and I expect the output will include
PKCS7 Encrypted data: pbeWithSHA1And40BitRC2-CBC, Iteration 2048
See if your curl has an option to specify the OpenSSL 3.0.x providers and if so specify (fixed) both ‘legacy’ and ‘default’. Otherwise, convert your pkcs12 like
# in 3.x.x
openssl pkcs12 -in old -nodes -provider legacy -provider default -out temp && openssl pkcs12 -in temp -export -out new
# or simpler
openssl pkcs12 -in old -nodes -legacy -out temp && openssl pkcs12 -in temp -export -out new
# in 1.x.x
openssl pkcs12 -in old -nodes -out temp && openssl pkcs12 -in temp -export -descert -out new
# and in either case securely delete temp; on systems with a memory tmpfs,
# typically /tmp, putting the file there can help assure this
# IFF 'old' was created by software that put the keybag before the certbag,
# which you can infer from the order displayed by pkcs12 -info,
# you can skip the temp file and pipe directly from one openssl to the other
# compare https://stackoverflow.com/q/54469754/95735 found by @PiotrDobrogost
Conversion loses any ‘friendlyname’ set in the existing file. For curl, and probably most other programs, this doesn’t matter, but if you want to use this same file with something where friendlyname does matter, add -name $name on the -export part.