Resolved
Overview of the issue
The following error message appears when your environment uses PHP 8.0. This error is not related to WPML, and may appear with any plugin or theme. It is caused by a translation error in the original MO file:
PHP Fatal error: Uncaught ValueError: Unknown format specifier ” ” in …/wp-content/themes/phlox-pro/auxin-content/options/auxin-options.php:10230
Stack trace:
#0 …/wp-content/themes/phlox-pro/auxin-content/options/auxin-options.php(10230): sprintf()
#1 …/wp-includes/class-wp-hook.php(307): auxin_define_options_info()
In this example, if you open …/wp-content/languages/themes/phlox-pro-fr_FR.mo file, you will find:
msgid “© %s. All rights reserved.”
msgstr “© s%. Tous droits réservés.”
As you can see, the translated value is “s%” while it should be “%s“
Workaround
As a workaround, remove the damaged MO file, and translate the string in WPML → String Translation.