@chrillek is unfortunately correct on this matter. And it’s also why we don’t advocate using a remote sync unless it’s really needed (and yes, in some cases it is).
Please, please do whatever maintenance is necessary to keep at least one of them working.
It’s not a matter of maintenance. If you ever read the release notes for DEVONthink you will see something regarding synchronization. We are always improving it and trying to employ different tactics to mitigate issues. However, as pointed out by @chrillek, as well as our documentation, and Sync Types Explained, there are inherent weakenesses in using a remote sync.
The issue is analogically no different than any delivery service. If you order a package from Amazon, they are dependent on the third-party contractors to deliver it on time. Even if Amazon switched vendors to someone promising better deliveries, things like weather, flat tires and mechanical breakdowns, traffic problems (very related to the issue at hand), etc. all can adversely affect when a package is delivered. None of that is Amazon’s fault nor are those things under Amazon’s control. But who gets the blame? Amazon.
Again, this isn’t about shifting blame, passing the buck, or pointing fingers. It’s about accepting the realities when using a remote sync option which is (1) not under our control nor (2) not under your control.
iCloud has been very unreliable in the past five months, including several outages. There has been quite the buzz in the Apple developer community about whether supporting iCloud or CloudKit is actually worth the effort given these problems. (And trust me, we’re not the only ones getting flak.)