LyX tries to open an additional "$$o" file in SumatraPDF

[Daniel]

I am using LyX with SumatraPDF. Following

https://wiki.lyx.org/LyX/SyncTeX#toc3
I use

SumatraPDF -reuse-instance "$$o" -forward-search "$$t" $$n

However, LyX tries to open an additional "$$o" file in SumatraPDF. Any 
idea what is going on?

Daniel