How can I update to Biber 2.8?

This should be fixed now - please get biber 2.8 from Sourceforge again.

I had exactly the same problem over here (Win7-64, TexLive 2017, updated yesterday).

Today, the invocation of tlmgr update --list reported an update for biblatex:

C:\Windows\system32>tlmgr update --all
tlmgr.pl: package repository http://ftp.cvut.cz/tex-archive/systems/texlive/tlnet (not verified: gpg
 unavailable)
tlmgr.pl: saving backups to D:/texlive/2017/tlpkg/backups
[ 1/12, ??:??/??:??] update: biblatex [7011k] (45690 -> 45702) ... done

... also 11 more packages, I removed from this answer ... 

running mktexlsr ...
done running mktexlsr.
running mtxrun --generate ...
done running mtxrun --generate.
regenerating fmtutil.cnf in D:/texlive/2017/texmf-dist
running fmtutil-sys --no-error-if-no-engine=luajittex,mfluajit --byfmt platex ...
done running fmtutil-sys --no-error-if-no-engine=luajittex,mfluajit --byfmt platex.
running fmtutil-sys --no-error-if-no-engine=luajittex,mfluajit --byfmt uplatex ...
done running fmtutil-sys --no-error-if-no-engine=luajittex,mfluajit --byfmt uplatex.
tlmgr.pl: package log updated: D:/texlive/2017/texmf-var/web2c/tlmgr.log

and this update solved the problem. Thanks for the question and thanks to Herbert for his comment.