The proceedings against Anna Lindstedt were scheduled to open on Thursday.
A Stockholm district court official said a new date was pending and could not comment on who had taken ill.
Lindstedt risks up to two years in prison if convicted.
She has denied wrongdoing in her efforts to free Gui Minhai.
Gui’s case has strained ties between China and Sweden.
In early 2019, Lindstedt organised a secret meeting at a Stockholm hotel with, among others, two businessmen representing Beijing, and Angela Gui, daughter of Gui.
As part of the deal to free Gui, Angela Gui was to stop criticising China in public, the charge sheet read.
The Swedish Foreign Ministry said it was not informed of Lindstedt’s move.
In February, a Chinese court sentenced Gui to 10 years in prison for illegally providing intelligence abroad.
The sentence was sharply criticised by Sweden, other countries and free speech groups.
Gui owned a publishing house in Hong Kong that sold titles banned in mainland China.
He disappeared in 2015 while on holiday in Thailand.
Early in 2016, he appeared on Chinese state television and admitted to leaving China without having served a two-year suspended sentence for his role in a drink driving death more than a decade ago.