Strange WiFi issues - interference?

Hi, I'm facing some very, very weird issues with our WiFi setup and I hope someone can help me out. For context: I'm in Germany. The basics: **Building layout:** 3 floors, 1 AP on the top floor, one on the bottom floor **Hardware:** Ubiquiti UAP AC Lite So, the thing is this. I had to reconfigure our infrastructure last Saturday because we went through a merger and I had to move some server gear from the old location to the new location, changed the IP addresses, everything was good. Tested the WiFi connectivity and stability on Saturday evening, and it was completely fine (got around 700 Mbps on a 5 GHz link on the upper floor, around 100 Mbps on the 2.4 GHz link). Then, at around 10PM on Sunday, I got pinged about WiFi break-ups and low bandwidth on the upper floor. I told a tech to reset the AP and I re-adopted it and deployed it again, and everything was good. Monday, 6:50 AM I got a call from our shipping department (bottom floor) claiming that the whole infrastructure is down and they can't connect to WiFi (due to building constraints, I can only have the access point cover that area). So I made my way to the location and tried everything I could think of. Switching channels, changing channel width, RF environment scan (which showed good) etc. The clients weren't even able to associate with the 2.4 GHz band, 5 GHz worked fine. The only thing that somewhat solved this issue temporarily was deploying an ancient Netgear access point that was collecting dust, giving them access at a blazing 30 Mbps with around 15% of packet loss. Upstairs, the situation wasn't much better, however now clients weren't able to associate to the 5 GHz band at all, and at 2.4 GHz, the bandwidth was at around 15 Mbps with a cool 40% of packet loss. I asked my tech and they confessed to me that they dropped the upper floor AP Lite from around ceiling height while resetting it... for good measure, I ordered two replacement units. I replaced the APs today and everything seemingly worked fine. However, now I'm home, and a couple of hours in, I got pinged again about issues with the top floor AP. WiFi experience in the UniFi Network application shows "Excellent". Interestingly, that same AP that was giving problems on the bottom floor worked flawlessly on the middle floor... What gives? I have already had a call with the Bundesnetzagentur (Federal Network and Radio Agency) for them to come out with a signal scanner, but it will probably be weeks before they can get to it. Unfortunately, I have a deadline on this of one week because some new hires will be moving in, I don't have the option of running cables, and I have a vacation booked from the 25th. So any help would be much appreciated. Due to time zone limits it might only be possible for me to reply in the morning, but I will do so. Thanks and great weekend to everyone!