Hm. Looks better now, I was able to upload most of them in batches of 10-15 each. But now it coughs up the same error. Nginx logs show an error 400:
[14/Feb/2019:11:49:51 +0000] "www.mysite.tld" 46.5.158.250 "POST /admin/customize/emojis.json?client_id=…3a4432c24effb&authenticity_token=…Bgz73aiLveyLUDkslPywsIKzrqb40O7Rul7h1iNAb6HVfjqO7wv1tVSiJnT4nL8wtBtvMvQ%3D%3D HTTP/2.0"
"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_3) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0.3 Safari/605.1.15" "-" 400 229 "https://www.mysite.tld/admin/customize/emojis" - 0.000 "-"(did not even know that there was an error 400…)