Create a new log directory exclusively for file-transfer-server, where log files must be stored.
This will likely fix selinux issues upon relabeling and logrotate.
Restorecon :
Launching restorecon on /var/opt/belledonne-communications/log recursively had the effect to relabel all the files. As /var/opt/belledonne-communications/log label was managed by Flexisip, under the label var_log_t, this caused issues with Apache and logrotate.
Apache was not able to write on the existing file-transfer-server log file re-labeled as var_log_t instead of httpd_log_t.
During logrotate, the creation of new files use the label of parent directory, causing the same issue as above. That's why we need the file-transfer-server to have its own exclusive log dir, which would be set to tag httpd_log_t instead of var_log_t.