Hi,
many thanks for your report. I guess you would need to change your last statement a little bit and specify complete path to the FreshRSS data directory
chown -R :www-data . && sudo chmod -R g+r . && sudo chmod -R g+w /opt/FreshRSS/data/
pls can you do a ls -la /opt/FreshRSS/ before and after you executed the command.