Exception 9 while using ESP8266 in both STA and AP

I start my ESP in both AP and STA mode. I expose a webpage with webserver on a softAP and list all available wifi networks in area. In the webpage you can choose a SID, fill a password and submit. After submission, the ESP should connect to that specific WiFi network, while still keeping the AP and webserver live. Once the ESP connects to the selected network, I get the follwing error:

Exception (9):

epc1=0x402028dc epc2=0x00000000 epc3=0x00000000 excvaddr=0x38393169 depc=0x00000000

ctx: sys sp: 3ffffdd0 end: 3fffffb0 offset: 01a0

stack>>> 3fffff70: 4021ca84 3ffee2c8 3fff3d2c 402028d4
3fffff80: 4021caaa 3fffdab0 00000000 3ffee518
3fffff90: 3fffdc10 00000000 3fff3d2c 4021fddc
3fffffa0: 40000f49 3fffdab0 3fffdab0 40000f49
<<

ets Jan 8 2013,rst cause:2, boot mode:(1,6)

ets Jan 8 2013,rst cause:4, boot mode:(1,6)

wdt reset

All code is available here:

https://jsfiddle.net/b570L2pn/