If you encounter this problem when reading csv-files, you might want to try to change the encoding from UTF-8 to latin1. This worked for me when using the tolower function in R.
Pretty nice solution here with gsub (https://stackoverflow.com/questions/38828620/how-to-remove-strange-char…):
gsub('[^\x20-\x7E]', '', text)
Similar info: https://www.r-bloggers.com/2012/01/removing-multibyte-characters-from-s…
Key words: multi byte utf-8 latin1