"Phonebook busy" - cannot save new contacts on SIM


Since a couple of weeks I cannot save new contacts on the sim anymore, but only on my phone. When I try to save them on the sim I get a message " the phonebook is busy, try later". I use the standard phonebook (did not download any app for that) and do not recall having had any specific problems or messages just before this change occurred.
Do you know how I can fix it? I’d like to have my contacts all on the sim rather than on the phone.
Thanks a lot in advance and apologies if it’s a silly question. Fairphone is my first smartphone :slight_smile:

Maybe there is not enough space on the SIM card? Wikipedia says that modern SIM cards can store up to 250 contacts. How many do you have on your SIM?