The hall is a former restaurant in the camp Olšovec. See #15 on the map below. It has one big room and one smaller, kitchen, bar and terrace with grill. Wireless internet connection is available in the whole camp, and there was a wired router/AP in the smaller room, so we could connect our equipment there. IIRC the connection was 20/20 Mbit/s. This hall is available only in the last week of August. (School holidays in CZ are July-August, so this is the last week of holidays.) If this term would not be acceptable for many people, we could use another room (also usually a catering room) in nearby building (16), but we did not like it that much.
Map of the camp
The main hall
The small room
(for other term than last week of august)
Backup backup hall
The camp on the other side of the lake (Tyršova osada) can be used as plan B or C - it has a hall and internet, but the place and staff did not impress me too much.