The cert for lemmy.sdf.org has issues. Not sure if it expired or what, but some apps report the key is unusable and only facilitate access after ~3-4 steps of authorizing the bad key. Some apps say the site is unavailable, full stop.. no option to continue.
Please do not authorize bad keys though. I know it's hard to resist for tech savvy people, because they want to know how the condition of the rest of the site is, but it could already be hosting malware or you're maybe about to get MITMed.