The general consensus online seems to be that this is caused by mod_sec which is an Apache security module. The basic fix seems to be to turn it off (via an email to your server admins). If it's only just started, I suspect that there's something changed in the Apache server configuration recently to either enable mod_sec or change its settings.
I asked Rochen to turn this off and immediately they did, K2 starting saving OK and some other issues I was having stopped. Salient and K2 are now working together like a dream, no problems at all.
Of course, I don't know if this is a similar problem to you, but it's worth sharing this solution just in case.
Regards
David