Hi Samuel,
I'm guessing its some kind of Org Mode vs Yasnippet issue where Org-mode is expanding it too fast, when it should wait for user input hence the "$1" section.
I guess yasnippet tries to indent the inside of the block (see =yas-indent-line=) before the lang part of the src block is specified. In Emacs 27.2, the default value of =org-src-tab-acts-natively= was changed to `t`. With this setting, trying to indent a src block with no language results in this error. -- Sébastien Miquel
