I kept on getting this problem for ages. I have loads of text files in csv format that I am reading in my program and the code for these files ie. 'LoadCSV(File.DirAssets' etc. is identicle to the file that B4A can not read and they are all read in perfectly so I know the installation, Java and folders were ok etc. I even copied a text file that worked to the directory and re named it to the one that didn't. It still din't work.
Eventualy I looked at the B4A ( the main program with the B4A extension ) file and the reason that B4A could not read one particular text file was because it had a space after the period (so instead of temp.txt it was temp. txt ). I just changed the file name to the correct one (without the space) within the .B4A main file and it worked.
Make sure you make a backup of the original B4A file before you manually alter it
I hope this helps anyone with the same problem.