No such problem here. Since you are mentioning squares, I assume you are making them with the rectangle tool. It automatically adds horizontal and vertical constraints. If you attempt to add a tangent contraint on two lines which already have a vertical/horizontal constraint, then you are over-constraining the sketch. Try to delete one of the vertical/horizontal constraints first, then apply the tangent constraint again.
Besides, please always report your FreeCAD info when reporting issues.
OS: Ubuntu 16.04.3 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.13289 (Git)
Build type: None
Branch: master
Hash: 23fff2b1d3a066d335dea509694e8f83f859d16a
Python version: 2.7.12
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.1.0
Locale: French/Canada (fr_CA)