When I open the model in RFEM/RSTAB, a yellow indication arrow appears on one of the supports. It disappears as soon as I click on the arrow or into the field of the sketch.
What does this arrow mean?
A yellow arrow in the graphic indicates the position that is selected in the results table.
This may be a support node, as it is in your case, or a grid point that is selected in the RFEM results table.
If the "Display" of tables is activated on the toolbar "Table" menu, and you click into a table row, for example in the results table 4.1 (RFEM), a yellow arrow is shown close to the node in the graphic so that you can find the relevant point faster.
This is the reason why the arrow appears on your screen after you have opened the structure, because a table row was selected in the results tables when the file was closed the last time, and this setting was saved.