Series of unexpected shut downs with DS1821+

My Syno has been super stable for couple years, and I've had it safely hooked up to a UPS, but a couple weeks ago it unexpectedly crashed and rebooted itself, beeping angrily upon reboot. It threw itself into read-only mode and told me to data scrub. I did a data scrub and converted back to read/write just fine, and made sure to get DSM up to date, hoping it was a OS issue. I contacted Synology support but all they did was suggest it could be my aftermarket ram (seems like easy blame, idk).

Since then it's twice slipped into battery UPS mode for a split second, sending notification of UPS battery mode then seconds later notification of reconnection to power.

Today it just did another "unexpected shutdown" and reboot. Specifically saying:

[Syno name] ran into a problem and was shut down improperly. This could be caused by power failure or other reasons and may result in severe data loss. Therefore, we highly recommend using an SNMP or USB UPS to protect your device and data if you don't already have one installed

This time it woke up not angry oddly, it initiated data scrubbing automatically but stayed in read/write.

Same as last time I was actively using NZBGet when crash occurred (which I use all the time), no idea if that's associated issue. The logs are not helpful at all, and don't seem to specify any blame for shutdown at all. Could it be the power supply? The RAM? UPS hiccups? DSM bugs? Would appreciate any help!