403 error when I try to save strings translation

Polylang was working perfectly until I moved my site from a subdirectory to the root, carefully following the procedure in the codex.

Now, when I click “Save Changes” at the bottom of the Strings Translation tab, I get a 403 error, Forbidden!

Things I have tried:
– disabling all other plugins
– re-installing polylang
– checking file permissions
– making sure polylang_mo_# entries in the db looked normal

the site is unionetudiante.ca

Please help!

https://wordpress.org/plugins/polylang/