question

Do you have a GX device? If so have you ve.direct connected them both to the GX device? It could be the multiples is drawing current, the batter voltage goes down from float, the charge controller kicks in with a bulk, inverter drops off, charge controller sees voltage rise and backs off, oscillating because neither can see what the other is doing...

@Stompke, welcome to the Community! I'll note a couple of things here:

First, your battery bank is massively undersized for the MultiPlus 3kVA; per the user manual, the 3kVA requires a minimum of 500Ah of battery bank underneath it. In the case of LiFePO4 batteries, since that capacity is entirely usable, you could get away with 300Ah, although general rule of thumb in sizing an LFP bank to an inverter is battery bank Wh = inverter Wx2, so the MultiPlus 3kVA, being rated to 2400w continuous, should have a minimum of 4800Wh of battery underneath it, which is ~400Ah. In practice, though, 300Ah of LFP is enough to squeak by as long as you're not going to be pushing the inverter to capacity... but under no circumstances is the single battery you have enough to support that Multi.

As a result, since you don't have the minimum requirements in place to support stable operation, it's definitely premature to blame the unit itself.

Second, however, please post photos of your installation itself; this sort of behavior is something I've seen multiple times before when the system is incorrectly connected, such as with other things than a battery attached to the MultiPlus DC I/O posts, undersized cabling, etc etc. Photos of the installation will help troubleshoot some of the basics there.

If possible, assuming that you have the Mk3-USB Interface for programming the unit, also send some screenshots of its settings as well, since there are several settings -including the maximum charge current- that would need to be dialed way down to account for the undersized battery bank.

I agree with @Justin Cook - Bay Marine Supply USA as to being undersized. However that's probably not what's causing your issue. My guess is the battery's BMS is tripping an over voltage condition. I don't know how lion energy handles over charged cells. These drop in batteries suggest a high charge voltage for lifepo4 so my guess is your are charging to 14.6v? then floating to 13.5v? I'd drop the charge voltage down to 14v and see what happens. Also if you ahve the multi maxed out at 120 amps, you're going to have a bad time as you'll probably trigger an over current with your BMS. This is another reason why one battery is not enough. Again like Justin said, post your settings from the multi.

I know why the drop in manufacture wants a hit voltage is so it has time to balance. However being at the top end of the charge voltage is too risky in my opinion with these cheap drop in batteries.