We have WPML on a multisite install of WordPress, and on the main site we have no trouble assigning translators. But when we go to one of the subsites and try to add a translator based on existing user, it doesn't find any of the users. These users are assigned to both sites and have no trouble logging in, but we just can't select them when trying to assign them as translators. Any idea what's happening there?
From the debug log you sent i could notice that you are using WPML 4.6.5, i would suggest to update the WPML plugins (v4.6.6 is the last version).
This issue should be fixed on the last update.
As a reference regarding this issue this is the errata link: