When installing AUR packages, it very often happens that I have huge problems with permissions.
Installed files are all from user:group root:root and have permissions set only on root (600 or 700). So I have to clean the mess before I can use anything at all.
I found the cause myself. I had the AUR Build Directory set to an mounted NTFS partition. WHen I moved the build dir back to /tmp/ everything works ok now.