latex runtime error with Xelatex compiler

environment: ubuntu18.04 + texlive-2020 + texstudio

I have successfully installed texlive and made sure by testing with latex -v ; xelatex -v, and they worked just fine. And in texstudio I finished configuring my texstudio with latex, pdflatex, xelatex and lualatex path.

Here is my latex code, which is very easy to read:

\documentclass{article}

\usepackage{xeCJK}

\begin{document}
    
    hello,你好
    
\end{document}

However when I compiling with xelatex, I came up with such error: (I build it with pdflatex compiler and it worked just fine without chinese.)

expl3.sty error: line 148

File ended while scanning use of \__sys_tmp:w. { \sys_load_backend:n { } }

Missing number, treated as zero. { \sys_load_backend:n { } }

Missing = inserted for \ifnum. { \sys_load_backend:n { } }

Missing number, treated as zero. { \sys_load_backend:n { } }

xeCJK.sty error:

TeX capacity exceeded, sorry [save size=80000]. \xeCJKResetCharClass

And I saw the snippet expl3.sty line 148:

{ \sys_load_backend:n { } }

With neither any parentheses error nor syntax error, how does this error come out?(The exceeding error maybe caused by other errors, I believed.) My latex build log:

(/usr/local/texlive/2020/texmf-dist/tex/xelatex/xecjk/xeCJK.sty
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3kernel/expl3.sty
Package: expl3 2021-01-09 L3 programming layer (loader) 

(/usr/local/texlive/2020/texmf-dist/tex/latex/l3backend/l3backend-xetex.def
File: l3backend-xetex.def 2020-01-29 L3 backend support: XeTeX

(|extractbb --version))
Runaway argument?
\q_stop <\__int_eval:w \c_zero_int \__int_eval_end: \exp_after:wN \use_ii:nnn \
ETC.
! File ended while scanning use of \__sys_tmp:w.
<inserted text> 
                \par 
l.148   { \sys_load_backend:n { } }
                                   
I suspect you have forgotten a `}', causing me
to read past where you wanted me to stop.
I'll try to recover; but if the error is serious,
you'd better type `E' or `X' now and fix your file.

! Missing number, treated as zero.
<to be read again> 
                   \__kernel_tl_set:Nx 
l.148   { \sys_load_backend:n { } }
                                   
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)

! Missing = inserted for \ifnum.
<to be read again> 
                   \__kernel_tl_set:Nx 
l.148   { \sys_load_backend:n { } }
                                   
I was expecting to see `<', `=', or `>'. Didn't.

! Missing number, treated as zero.
<to be read again> 
                   \__kernel_tl_set:Nx 
l.148   { \sys_load_backend:n { } }
                                   
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)

)
Package: xeCJK 2020/10/19 v3.8.6 Typesetting CJK scripts with XeLaTeX

(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/l3keys2e/l3keys2e.sty
Package: l3keys2e 2020-10-27 LaTeX2e option processing using LaTeX3 keys
)
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xtemplate/xtemplate.st
y
Package: xtemplate 2020-10-27 L3 Experimental prototype document functions
\l__xtemplate_tmp_dim=\dimen139
\l__xtemplate_tmp_int=\count181
\l__xtemplate_tmp_muskip=\muskip16
\l__xtemplate_tmp_skip=\skip49
) (/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse.sty
Package: xparse 2020-10-27 L3 Experimental document command parser

(/usr/local/texlive/2020/texmf-dist/tex/latex/l3packages/xparse/xparse-generic.
tex))
\l__xeCJK_tmp_int=\count182
\l__xeCJK_tmp_box=\box47
\l__xeCJK_tmp_dim=\dimen140
\l__xeCJK_tmp_skip=\skip50
 (/usr/local/texlive/2020/texmf-dist/tex/latex/ctex/ctexhook.sty
Package: ctexhook 2020/10/19 v2.5.5 Document and package hooks (CTEX)
)
\g__xeCJK_space_factor_int=\count183
\l__xeCJK_begin_int=\count184
\l__xeCJK_end_int=\count185
\c__xeCJK_CJK_class_int=\XeTeXcharclass1
\c__xeCJK_FullLeft_class_int=\XeTeXcharclass2
\c__xeCJK_FullRight_class_int=\XeTeXcharclass3
\c__xeCJK_HalfLeft_class_int=\XeTeXcharclass4
\c__xeCJK_HalfRight_class_int=\XeTeXcharclass5
\c__xeCJK_NormalSpace_class_int=\XeTeXcharclass6
\c__xeCJK_CM_class_int=\XeTeXcharclass7
\c__xeCJK_HangulJamo_class_int=\XeTeXcharclass8

! TeX capacity exceeded, sorry [save size=80000].
<argument> ..._xeCJK_begin_int =\l__xeCJK_tmp_int 
                                                  \int_incr:N \l__xeCJK_begi...
l.656 \xeCJKResetCharClass
                          
If you really absolutely need more capacity,
you can ask a wizard to enlarge me.

 
Here is how much of TeX's memory you used:
 973 strings out of 477220
 23745 string characters out of 5830450
 300467 words of memory out of 5000000
 20745 multiletter control sequences out of 15000+600000
 403438 words of font info for 28 fonts, out of 8000000 for 9000
 1348 hyphenation exceptions out of 8191
 67i,0n,75p,234b,80001s stack positions out of 5000i,500n,10000p,200000b,80000s
No pages of output.

So my question is:

1. what is \sys_load_backend, is it a keyword?
2. Is expl3.sty file broken? If it is, how can I fix it?
3. The worst condition: Do I need to reset texlive? Though I have done this twice :(