Most likely the assertion fails on your SDK workspace path. Workspace path defaults to your user home directory unless you change it either during SDK installation or later in the IDE through Options > Sailfish OS > Build Engine > Workspace folder, where you can also check the currently used path.
If the assertion fails, then it means your path contain some redundancy like extra "." or ".." components or extra path separators (e.g. multiple leading slashes).
If you do not see anything like that in the UI, then I would suggest you to temporarily change the path and set it back, using a clean path.