wrong "insufficient space" message

When I try to install the SDK in /opt/gui/QtSDK it fails with a message saying "insufficient space (...) requires 1.1GB". The problem is, that on the volume where I have /opt/gui/QtSDK there is 262GB free space.

Note: /opt/gui is a symbolic link to /home/system/opt/gui (still there is 1.9GB free on /opt itself - but anyway the SDK should go to another volume, the one with >200G free...)
See attachment for the filesystem details.

(Edit) It installs with no problems directly to /home/system/opt/gui , but not to /opt/gui (which is the same = symbolic link).