Hello
Solved!
Recompiling the form which called the one with the error solved the issue. It is working OK again.
Apparently, this was a migration issue: it was working on VFP8, and when migrating to 9 some forms were not recompiled.

BTW, I have been using member arrays of objects in forms for a long time, always thought they were a standard feature...

thisform.Enc(i).move((i-1) * 50 + px, py)
thisform.Enc(i).visible = .t.

Thanks!