Symptom: Using "Add Library" adds the #include in the code, but no library-node is created in Solution Explorer.
Afterwards Build/Compile/Upload work fine.

Same issue with existing projects, as well as new ones (empty or Blink).

This has been working fine for years until some 2 weeks ago (and I noticed it some days after I moved my libraries and code).
Libraries and Code are all under folder Arduino
[Arduino]
  - [libraries]
  -- [AccelStepper]
  -- [more libraries]
  - [Project 1]
  - [Project 2]

I moved the entire Arduino-folder one level deeper and changed
VMicro -> General -> Configure IDE Locations to reflect the new situation.
And since compiler can still find the libraries I assumed this was done right.

Eventually, 
Uninstalled and re-installed Visual Studio en Visual Micro
Win 11 Pro, 25H2, OS Build: 26200.9168
Visual Studio = 17.14.39
Visual Micro = 2026.708.2204
Did not solve it.

Additional info:
- An existing project showed only nodes for those libraries that had vcxitems-file in the library-folder.
- Manually creating a vcxfile in a library-directory and hacking that into the.sln-file made the library appear as a correct node (with cpp and h below it) in Solution Explorer
- "Add library"-Refresh shows "Toolchain reload complete". No error messages afaik.
- File vcxitems is not created either on toolchain reload, "Add Library" or compile/build
- Started VStudio with /log (and added the lib using "Add Library"), but cannot deduct any error message from it
- As per your request I added the Build-log, although I assume it's not build-related

Any suggestions?

Thanks for the report.

If a library contains a VCXITEMS file then it is seen as a Shared Project, which can be referenced in the current solution.  This can be better for intellisense but can have issues when moving PCs with paths etc..

If you normally saw this in the past then it is likely that you had one of the vMicro > Add Library > Library Advanced options enabled (e.g. Create Shared Project when Including Libraries)
Add Library Documentation: https://www.visualmicro.com/page/User-Guide.aspx?doc=Add-Libraries.html

The libraries can also be cloned into the project/solution using these options, and an alternative for complete isolation and version pinning is the Portable Solutions approach available.
Portable Solutions Documentation:https://www.visualmicro.com/page/New-Arduino-Portable-Solution.aspx

Does that help?