actually I was facing the same problem on my live site, I just unstall kunena, cleared administrator and site cache and reinstall again, and then my problem solved, so I shared my experience with you.
Thanks for your response.
The problem is on my local server, because I just found out that mbstring in php7 on Linux Mint is installed separately.
After I'm searching, the problem 500 internal server error cause mbstring, I found the solution here:
Maybe, someone have the same problem.
And for that matter kunena_v5.1.0-RC1_2017-12-31, as Albirich Enric said to be waiting for the full version.
*thanks, and i'm sorry if my english so bad.