I have reinstalled Rtools (now it's just in c:\Rtools\); from R help, I was told that it was the problem of devtools package. When I try to execute something like devtools::install_github("r-lib/devtools"), I get another error message:
Error in system(full, intern = TRUE, ignore.stderr = quiet, ...) :
running command '"C:/PROGRA~1/R/R-35~1.0/bin/x64/R" --no-site-file --no-environ --no-save --no-restore --quiet CMD config CC' had status 254
And when I try to run it manually, by launching c:\Rtools\bin\sh.exe and then executing any command, I get bunch of errors like this:
1 [main] sh (12212) C:\Rtools\bin\sh.exe: *** fatal error - cygheap base mismatch detected - 0x32E5410/0x3285410.
This problem is probably due to using incompatible versions of the cygwin DLL. Search for cygwin1.dll using the Windows Start->Find/Search facility and delete all but the most recent version. The most recent version should reside in x:\cygwin\bin, where 'x' is the drive on which you have installed the cygwin distribution. Rebooting is also suggested if you are unable to find another cygwin DLL.
and I couldn't find any other cygwin1.dll anywhere...