When I upgraded from Pega Robot Studio 8.0 to Pega Robot Studio 19.1 and rebuilt it, the following warning message was output.
C:\Program Files(x86)\MSBuld\14.0\bin\Microsoft.Common.CurrentVersion.targets(1820,5):warning MSB3245:Could not resolve this reference.Could not locate the assembly "log4net, Version=22.214.171.124, Culture=neutral, PublicKeyToken=1b44e1d426115821". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.
Looking at the reference property of log4net, the path is as follows, and I suspect that it cannot be referenced due to the "\\" part.
@ToshikiSThe references are listed in the osproj file. You could edit that with the solution closed, assuming you have made a backup. I would just delete the reference and re-add it. All the stock DLLs should be in the installation folder.