I have a problem with items disappearing or with DnD beyond "resetting" my money and equipment. I use DnD beyond with chrome on my android smartphone.

I keep adding equipment to my list, sometimes normal stuff from the list, sometimes custom items. Then I keep using the side, or go to another side. When I return later, my changes are reset. The items are not in the list anymore and my money is reset to the original amount. I then add it again, sometimes two or three times until it works. Is there some way to "save" my changes? Or is there something else I''m doing wrong?

It really costs my a lot of time especially with custom items and sometimes it can even happen without me noticing or maybe not remembering what an item was worth or how much gold I've spend. 

Does anybody know the reason for this happening?

Because the server cache is holding old information. Log out, log back in, wait 20 minutes, and then try it again. That will force the server’s cache to reset. After that you should no longer have the problem.