I am loving Dolibarr so far, however, I always have issue when doing a manual upgrade from version 7.03 to 8.02.
Dolibarr 7.03 is installed via Installatron on a Plesk hosted server, what I did was unzip all files and replace all files on the existing folder.
After that, I ran the upgrade wizard and ended up with a redirect loop to install.index.php, not able to login anymore although I deleted install.lock and install folder.
We have a lot of problem with Plesk. It seems Plesk make uncommon things into web server application deployment but i don’t know Spek so i’m afraid i can’t help a lot.
At least, check that your htdocs/conf.conf.php contains correct information (Plesk may have corrupt them)
Check also that the web server has permission to read the file.