I have just tried this with one of my own listings and did not have any trouble changing the title.
It may be that it is a problem that only a few sellers are encountering and Ebay are sorting it out as members complain.
It's probably worth contacting Ebay directly to report the problem as it is affecting your account.
Use the link below to get an online chat or callback. Maybe have a specific item number to hand so they have something to look at.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)