Try one of these, it worked for me the last 10 days on various connection errors from my 7 hosts. The documented issues of server move, new ISP and DNS problems plus the checksum file errors could all be wreaking havoc.
1) One workaround is to use the BOINC Manager proxy settings, using a URL of http://boinc.rieselsieve.com and a port of 8000. If you are attached to multiple projects, make sure you suspend them before updating and then turn off the proxy & resume the other projects once you are done.
2) Step 1 is temporary fix to u/l finished RS work units and report. Once you have cleared any finished work, try detaching from RS, rebooting your PC and reattaching.
Goodbye Windows 10. R.I.P. -Rust In Pieces.