It may not be the most ideal way to resolve this, but I was able to avoid that error by converting the tab separated .raw file produced by plink2 to match the space separated .raw file produced by older versions of plink.

I used the tr command, but I'm sure there are many better ways of doing this with awk, sed etc..

cat data1.raw | tr "\t" " " > space_data1.raw

Afterwards you can run read.plink in R as usual using the new file.