Looks like Synology devices are replacing valid certificates installed by users with their own revoked 'synology.com' one after each reboot.<p>If you have HSTS enabled, the only way to access the NAS is to disable checks which puts you at risk of MITM attacks.
FWIW i've just verified this on a DS1019+ running the latest DSM. My certificate disappeared after rebooting and the default synology one (which I had deleted) reappeared.