First signs of a tree dying? Difficult to say, before dying a tree shows stress, the tree is dying when it does not recover from stress. If the tree turns brown and black, and it is not autumn, it is dying. Give your Acer ginnala time to recover. Relax. It may look stressed, but if you keep trying to do something else with it, you will aggravate the situation, make it worse. Just give your tree "good care", sun, especially morning sun, afternoon shade, and keep a daily check on whether it needs water. And it should recover. Don't try to do anything more than give it time to recover. The danger is you will "help it to death". Just put the tree on your bench in the back yard, and leave it alone, beyond watering as needed.
I do a lot of summer repotting of conifers. Works well in my specific climate. There are a few deciduous trees I repot in summer, but only a few. Generally, deciduous are best repotted late winter, early spring. Generally. There is a HUGE climate factor that determines the feasibility of repotting in summer. I do not know the climate in Slovenia. I have a guess, but do not know. Summer repotting of deciduous trees is NOT FOR HOT SUMMER CLIMATES. Where I live, and Sorce lives, we have relatively cool summer climates. We average fewer than 10 days per year with highs over 32 C (90 F). That is considered a mild summer region. If you have a large number of days in summer over 32 C ( 90 F ) summer repotting is not as good a solution. Climate is important.
I still repot the majority of my deciduous trees in EARLY SPRING, just before buds swell, or as the buds are swelling, before buds open. Sorce and myself are in the same general climate zone. Summer repotting is not a blanket solution to repotting problems. Repotting deciduous in summer can be done, but it is an "advanced topic" in that it is not always simple or obvious what all you need to do, to get the tree through the process. Sometime you need to prune back foliage, sometimes not, sometimes it might be wise to defoliate at the time you summer repot, sometimes not. It is just too complicated a topic to tackle without extensive photos and pages and pages of typing.