That particular instance was very recently the source of a lot of CSAM and spam, so that’d be why. A lot of instances recently upped their security to combat that.
Just to add some more context, there was an attacker recently who created accounts on several Lemmy instances and used those accounts to spread CSAM. On lemm.ee, this attacker created 4 accounts over a 24h period, but was not able to upload any CSAM to our servers due to our stricter upload rules (we require 4 week old accounts to upload any images at all), and all of the 4 accounts were removed very shortly after creation (most of them within an hour of signing up). The attacker gave up trying to use lemm.ee very quickly, and moved on to other instances.
I just wanted to share this context to illustrate that while indeed the different measures we implement to protect the instance can have a negative impact on legitimate users, I really believe that overall, they have a net positive effect. In addition to Cloudflare DDoS protection and image upload restrictions, we also have a separate content-based alerting layer on top of Lemmy, which allows our admins to quickly notice when something suspicious is going on. As another example, this alerting has allowed us to extremely efficiently deal with a current ongoing spam attack on the Fediverse, and I bet many lemm.ee users aren't even aware of this attack due to the quick content removal. We will continue to improve our defenses, and hopefully try to limit the impact on real users as much as possible, but some trade-offs are necessary here in order to protect the overall userbase.