Permission problems after last Catalina update on iMac

I have recently updated my iMac to Catalina and immediately had the problem that I was asked for my admin password for pretty much every move, e.g. moving files from one folder to another ( no need to stress howe annoying that is when you have to do this hundreds of times). I fixed this problem by clicking on "get info" for the Macintosh HD and allow me as an admin to "read and write". After the latest system update of Catalina, the same problem showed up again, I am prompted for my admin password to move files. But now things got worse, as I typed in my password and got the reply that I don't have permission to move my files. I tried to change my admin permission for Macintosh HD as I did before, but this time I was told again that I don't have permission to do this (despite the fact that my password was good enough to open the padlock to make changes). Any idea how to get rid of this bug? I don't want to be asked for my password all the time, especially if the reply is that I don't have the permission. I have already checked in the System preferences, I do have admin rights. However, I noticed that my user name on the list in System preferences is slightly different from the admin name on the Mac HD (1st and surname written in one word versus two different words). Might that be the problem, and if so, how do I fix it (and I have no idea where that change of names would have come from). Thanks for your help!