Unfortunately, this module is susceptible to a [link|https://www.anandtech.com/show/11110/semi-critical-intel-atom-c2000-flaw-discovered|design flaw within its CPU] that is likely to eventually render it inoperable. [link|https://community.synology.com/enu/forum/1/post/131830?reply=505293|There is a temporary fix that involves soldering a 100ohm resistor to the motherboard], but that will not permanently fix the problem, just buy you more time. I would recommend pulling your drives before attempting a fix, and using a new drive for initial testing after any attempted repair.
Alternatively, your original drives / storage volumes should work with no issues if installed in another newer Synology product, which you may want to start saving for even if you're able to temporarily revive the DS1815+ via the resistor fix.