Somehow this thread got shanghaid / broken, but here's something hopefully useful to anybody hit with this hammer: 95% through my latest translation
a) I can not proceed translating
b) I can not back out (Translation side grayed)
c) I can not create partial result - clicking create translation causes the above error to appear.
Duh!?... Fact is, it happened the second time around on the same article. The first time around I avoided the obstruction by copying source to the sandbox, deleting the questionable infobox and restarting(!). It did not take me long to end in the same impasse. However, this time it is not some box - and probably never has been -, it is the environment of the wikipedia (slovenian) I am working in. Here the javascript console printout - after the message above:
Gadget "Navigation_popups" was not loaded. Please migrate it to use ResourceLoader. See <https://sl.wikipedia.org/wiki/Posebno:Vti%C4%8Dniki>. Special:ContentTranslation:1:45
Gadget "refToolbar" was not loaded. Please migrate it to use ResourceLoader. See <https://sl.wikipedia.org/wiki/Posebno:Vti%C4%8Dniki>. Special:ContentTranslation:1:215
.... etc ...
This page is using the deprecated ResourceLoader module "es5-shim".
Use of the "es5-shim" module is deprecated since MediaWiki 1.29.0 load.php:1:74
... etc ..
It all looks like my wikipedia enviroment is not what the SpecialContentTranslation expects it to be.
At the very least it should not be deprecated >:/