It may be cause that the file upload is not completed definitely. After having confirmed FTP transfer mode, upload once again.
In addition, there is the method to upload zip file to your server.
Upload zip file to server.
Login by SSH.
Move all unziped files to document root(This may be unnecessary depending on way of unzip).
More, it may be cause for some reason that most important constant (QA_BASE_DIR) of Q2A is not set definitely. Insert below codes at qa-include/qa-base.php (L150). And check QA_BASE_DIR(absolute path).
echo 'QA_BASE_DIR = ' . QA_BASE_DIR;