BaturalpS,
On behalf of Tom's Moderator Team, welcome aboard!
You have a flow problem due to a blockage, which is typical for a 5 year old AIO. Many AIOs fail well within that time frame. It's very likely that the blockage occurred when you removed your H100i v2 and reinstalled it in your new case.
Apart from the potential for a coolant leak, there are several problems common to AIOs: Here's some information you should know about AIOs, but pay particular attention to the double red bullets below:
• All AIOs
will eventually fail. It’s not a question of
if; it’s a question of
when. Pumps can have component failures in the stator electronics that drive the impeller, which is a moving part that can wear, so those which run 24/7/365 are prone to premature failure. AIO units are notorious for failures due to inferior pump quality, whereas custom loops typically use high-end pumps which have greater longevity.
• Coolant can slowly evaporate over time due to "permeation". However unlikely it may seem, coolant can actually dissipate directly through the tubes. This can introduce bubbles into the impeller chamber causing "cavitation", whereby coolant flow is impeded or interrupted.
• AIOs are sealed Closed Loop Coolers (CLC), which unlike custom loops, are not designed to be disassembled for the water block and impeller to be flushed, cleaned of bio-contaminants and radiator sediments, then refilled. Even AIOs that have an effective biocide additive gradually accumulate bio-growth over time, which will eventually clog the micro channels in the water block, as well as the radiator cooling tubes, thereby reducing flow and thermal efficiency. This is evidenced by Core temperatures that slowly increase by several degrees over periods of several months to a few years.
• Also unlike custom loops, AIOs use dissimilar metals (aluminum radiator / copper water block). This causes galvanic corrosion which produces sediments that accumulate over time, resulting in blockages and flow problems.
Even new AIOs may contain radiator sediments due to inadequate flushing after manufacturing. Experienced builders of custom loops will always thoroughly flush brand-new radiators. Flux, solder and metal fragments are typically found in the flush water when it's poured through a strainer.
• • For no apparent reason ... OR ... by simply reinstalling, moving, tipping, handling or otherwise disturbing a NEW OR OLD unit, particles can become dislodged, whereupon the next power-up, the now free-floating particles can circulate into the impeller chamber, micro fins in the water block, or into radiator tubes, thereby constricting or completely stopping coolant flow. Since the impeller is magnetically coupled to the stator (no direct shaft), the unit may "appear" that it's running while no actual flow is present.
"Hearing" the unit running or "feeling" vibration can be deceiving as it does not necessarily indicate flow, nor does Pump RPM in BIOS or various software utilities. Moreover, as fan vibration can "telegraph" throughout the entire AIO unit, it's often mistaken for pump vibration.
• Without an actual in-line sensor or indicator, proper flow is difficult to confirm. Under normal operation, even with the CPU at 100% workload, there should be only a minimal temperature differential between the tubes. However, if one tube is hot while the other is cool, or the water block is hot while the radiator is cool, it indicates little to no flow. Surface temperatures can be verified with an infrared (IR) thermometer.
Since your AIO is 5 years old, it's highly likely that bio-contaminants and particles from galvanic corrosion have impeded coolant flow as they circulate throughout the unit.
Fortunately, Corsair has a five year warranty, which, much to their credit, they honor without difficulty. I suggest that you submit an RMA request for a warranty replacement cooler. In the interim, a backup air cooler should suffice. While your AIO remains in service, be sure that you don't leave your rig powered up or unattended when not in use.
Once again, welcome aboard!
CT 
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