On this new machine, every time I try to insert a ToC I get the mentioned error: idxexample.odt is locked for use on a different system since". Then I have the choice to open it read-only (which it is anyway, due to permissions), Open, or Cancel. Opening it leads to a normal ToC insert.
The Win8.1 user has Read and Execute permissions on the file (at program files (x86)/libreoffice 5/share/template/common/internal.
I’ve tried a new user profile - no difference.
I’m on LO 5.2.0.4 x32, on Win 8.1 x64.