The building service seems to be in a jam currently. Hopefully this manual solution would be helpful for somebody.
Personally I have installed a few versions manually on my Linux Mint 19.2 Cinnamon (based on Ubuntu Bionic/18.04).
It does involve a series of operations that some may find difficult, such as:
1. downloading all files pertaining to the desired release, both i386 and x64, according to the Linux OS version from here
2. manually opening the .deb > data.tar.xz files (through the archive manager) and copying the folders from ./opt/wine{-devel/-staging} to their corresponding folders in ~/.PlayOnLinux/wine/linux-amd64/{wine_ver} and ~/.PlayOnLinux/wine/linux-x86/{wine_ver} respectively
That's all. When opening Tools > Manage Wine versions from POL's main menu you'll see the desired version already installed in the right-side panel.
Now let's give a practical example for a better understanding. At the time of typing this there is Wine 7.0~RC2 available but in POL there's nothing above 6.18 in the x86 section and 6.17 in the x64 section. I've been following the devel branch lately so I installed this 7.0~RC2 devel version.
- create a folder named i.e. Wine7.0 somewhere in ~/Downloads or wherever you see fit
- download there all x64 devel_7.0~rc2 files from the Wine repository here (except for those containing dbg or winehq in their names)
- download there all x86 devel_7.0~rc2 files from the Wine repository here (except as above)
- create two folders: ~/.PlayOnLinux/wine/linux-amd64/7.0 and ~/.PlayOnLinux/wine/linux-x86/7.0
- open each of the downloaded packages one at a time (through Archive Manager, not GDebi), select all available folders within ./opt/wine-devel/ and drag them to their corresponding folders created earlier - 32bit to 32bit folder, 64bit to 64bit folder
If everything went well there will be four subfolders in each 7.0 folder: bin, include, lib64, share in the amd64/7.0 folder and bin, include, lib, share in the x86/7.0 folder - each with their own files. Now opening Tools > Manage Wine versions from POL's main menu you should see version 7.0 installed for both 32bit and 64bit Wine. Proceed with creating your prefix(es).