Jindrichus: I have got very similar problem, although the error message is a bit different:
Error: Invalid character after parsing property name. Expected ':' but got: e. Path 'm_MainState.m_EntityData[332].UniqueId', line 1, position 5537792. at Newtonsoft.Json.JsonTextReader.ParseProerty () [0x00149] in <4f213745d60e425fb763df319c826ddf>:0
It also permits me to enter the Old Sycamore. I could not solve the Jaethal's quest. It was annoying, but sufferable. But now it is about Linzi!
I am playing Linux GOG 2.1.5d version. Is there any workaround to get to the Sycamore or at least some way to edit the savegame and cheat the quest solution?
No idea about "legit" fixing your savegame (maybe there're some editors), but if you want to cheat your way through bugs you can use mod called Bag of Tricks. It's basically a in-game console with QOL features, but also cheats including quests parameters. So I guess it might help.