How to reproduce:
1. Create a folder named "Пример"
2. Create and open a new Unity project in it
3. Open the Preferences windows by clicking Edit > Preferences
4. Go to the External Tools and select Visual Studio as the External Script Editor
5. Create a new Script by right-clicking in the Project window >Create > C# Script
6. Double click the newly created Script

Expected results: the Script opens together with the .sln file
Actual results: the .sln file fails to open

Reproducible with: 1.0.2, 2.0.2 (2020.1.0a15, 2020.1.6f1, 2020.2.0b3, 2021.1.0a1)
Not reproducible with: 2018.4.27f1, 2019.4.11f1 (1.0.11), 2020.1.0a14 (1.0.2, 2.0.2)

Notes:
-The full error message states: "These files could not be found and will not be loaded"
-This issue doesn't reproduce with VS Code or JetBrains Rider
-This issue doesn't seem to be dependant on the Visual Studio Editor package version
-The issue doesn't reproduce on any version when the Visual Studio Editor package is removed