TinyMCE is broken

[snobo]

After upgrading to version 1.6-git, TinyMCE is broken - only HTML is displayed, there are no buttons in the toolbar except for "HTML" (which doesn't switch to formatted text), and in the console the following error is output:

jquery.min.js?s=1653903095:36 Uncaught Error: editor.addButton has been removed in tinymce 5x, use editor.ui.registry.addButton or editor.ui.registry.addToggleButton or editor.ui.registry.addSplitButton instead
at JE.addButton (tinymce.min.js?s=1624418460:9:383133)
at Object.a.config.setup_callback (ui.min.js?s=1609105339:45:319)
at JE.conf.setup (editor.js?s=1653903053:167:12)
at JE.execCallback (tinymce.min.js?s=1624418460:9:378795)
at new JE (tinymce.min.js?s=1624418460:9:384959)
at n (tinymce.min.js?s=1624418460:9:387585)
at Array. (tinymce.min.js?s=1624418460:9:389249)
at Ce (tinymce.min.js?s=1624418460:9:5625)
at Object.e (tinymce.min.js?s=1624418460:9:389108)
at Oi.executeHandlers (tinymce.min.js?s=1624418460:9:67058)

[alecpl]

Strange.

roundcubemail/skins/elastic/ui.js

Line 1406 in ca26477

ed.ui.registry.addButton('plaintext', {

An old version of ui.min.js is used somehow. Note that by default there are no min.js files.