Hello Jos!
The Orchestral Strings are looped and shouldn't drop out. That's why I thought that you may have loaded the Solo Strings. I can only guess why this is happening. Maybe the two violins are playing on the same channel. Please make sure that you have reserved a program and channel for both sections on the Sibelius Manual Sound Sets page and in Vienna Ensemble. If you have made any output settings in the Sibelius mixer, please reset all outputs to (Auto).
Best,
Andi
Vienna Symphonic Library