CLI problem? warning Backends flushed.

[_maibi]

SDK: 15.3.0

I have developed PCA10056 board based on CLI example.

I tried to print debug strings.

but, [00: 00: 32.310,668] <warning> app: Prints backends flushed repeatedly.

and then, uart_cli:~$ input line is disappeared and dead.

how can I solve Backends flushed warning?

at dead, gdb where full - 

#0  0x00007ebc in app_usbd_string_desc_get (idx=0 '\000', langid=0) at ../../../../nRF52_SDK/components/libraries/usbd/app_usbd_string_desc.c:291
        lang_idx = 0 '\000'
        str_pos = 2 '\002'
#1  0x00000000 in ?? ()
No symbol table info available.

[yu1998]

I'm wondering if i enter too fast the cli will stuck and cannot enter anymore how do I fix that