After updating to 1.9.7, I encountered an error when uploading files on all my WordPress sites: the temporary folder was not found. What could have caused this?
You should check the log:
If using Nginx the log is here: /var/log/nginx/domains/YourDomain.error.log
If using Nginx+Apache2 the log is here: /var/log/apache2/domains/YourDomain.error.log
Hestia creates the php pool for your domain setting upload_tmp_dir to /home/YourUser/tmp.
grep -r upload_tmp_dir /etc/php | grep YourDomain
Check that the dir exists and you have free space.