After seeing at client-side this error message:
EN: "TCC cannot suggest a position for this student. Please contact the TCC operator to free up a position."
FR: "TCC ne peut pas proposer un poste pour cet étudiant. S'il vous plait contacter l'operateur du TCC pur libérer un poste."
You can solve this issue by following these steps:
1. login to the TCC as admin
2. on the left hand side you now see a new option:
"Edit class layouts", you need the "Identify student computers" icon (black)
3. Free-up positions by selecting the position you want, and then delete them (red cross), as illustrated in this screenshot:
3. now it should be possible again to identify new student PC's..