QFontDatabase::loadFromCache: Could not open font database cache!

I got this error.
On my device a QWSServer application is running.With it I am trying to run my own application.My application needs this server so there is a client-server structure.But I got this error.
QFontDatabase::loadFromCache: Could not open font database cache!

Do you happen to have any static QFont variables?

No I have not.My application was working correctly before creating QWSServer application.QWSServer is a keyboard driver and is working continuously.

Was this server started by the same user as your application? Maybe it's matter of access rights?

Qt seems to create the font db in the /tmp directory. Make sure that you have space on this device. An application will run with '-qws' as it creates the fonts (to render in display pixel depth, etc), but clients are expecting to load the glyphs from a cache.

$ df
...
rwfs 512 512 0 100% /tmp

$ mount -o remount,relatime,size=4M /tmp
$ df
...
rwfs 4096 512 3584 13% /tmp