Sorry for taking forever to reply, I managed to get the error message again, and this is what it said:
file://
"ERROR, saving failed. The browser reported the following message. QuotaExceededError: The quota has been exceeded.
This likely means the domain is out of storage space. In order to save, clear this domain's 'local storage', or use the save download/upload feature instead"
After getting the message I can delete old savefiles again which was really weird, but it took me deleting 4 savefiles in order to be able to save again. I am the kind of person who likes to save before every fight, so maybe that was my issue lol. I guess I could limit myself to 3 save slots and download each batch as separate savefiles. Thanks anyway!