From searching the forums, it seems the invoice numbering rules have changed, or are being more strongly enforced.
I have always used spaces, eg INV A00001 but I now cannot do this. I cannot replace the space with a dash either.
I am not against changing the format of my invoice numbers, but would prefer to do this at a time that makes sense - eg at the beginning of a new accounting period, not half way through.
If you’ve always used this format, you should be able to just create a new invoice and the numbering will pick up from the last one and automatically increment it - there wouldn’t be a need to manually change it.
If you’re not seeing this (e.g. an error when trying to save), please let me know and I’ll take a look for you.
I have a draft invoice (its been sitting there a while) and I edited it today with a new line & got the "must be alpha-numeric warning). The only way to save it was to remove the space.