We too have this problem on Homepage of our customers site. It is currently on the business plan, and with 12 collections on the frontpage. But it will not allow us to add anything anywhere. On all other pages of the site we still can.
A little debugging showed, that If we remove 5 collections, so we are down to 8, we can add again.
Mind you it is NOT just collections we cant add. It is anything… divs, headings…
I sent the share URL to support, but due to agreement with the client, I can’t share it here unfortunately.
Hope for a quick fix.