I found a solution for a setup challenge and I would like to share it in case others have a similar issue.
I had to connect 2 DAWs - one situated on stage, one in the back of the audience room. Originally, I used 2 Toslink cables, but with lengths exceeding 5 meters, these connections became unreliable and I started to get crackles - and I need to cover a distance of 10 to 40 meters. I tried out many Toslink cables, but nothing works reliably.
I looked at Toslink extenders, but I was never convinced. It seems that most of them are consumer grade stuff and not studio material. And the real high-end solutions like MADI are way out of our budget.
Then I stumbled across the small Swiss company AppSys ProAudio (http://www.appsys.ch, a one man show) while on tour in Zürich. The guy engineers small, affordable ADAT extenders. After a technical conversation with him, I had the impression that this man really knows the issues and what he is talking about.
I tested his 16 channel ADAT extender (bi-directional ADAT, 1 ADAT pipe per direction). The extender uses regular CAT5 cable up to 100 meters in length. I am connecting 2 rme Multiface II with it. The DAW on stage gets it's clock from the other DAW via ADAT though the extender.
I am pleasantly surprised - it works perfectly. The pair of extenders is 160 Euros - less than a set of high-end Toslink cables I tried out.
May be this spares others from wasting lots of time and money.