I use 1234 as a port, so could the particular port be a problem？
e.g., www.mydomain.com:1234/survey

If it is a problem, how to solve it?

I can share the access of LS. It will help many users who used the particular port too if we can fix the issue.

Thank you so much for your patience

No idea,

Maybe a kcfinder issue or something else.
You can try to report the issue for starting : community.limesurvey.org/bug-tracker/

Else : if it's urgent : www.limesurvey.com/support

It will help many users who used the particular port too if we can fix the issue.

My rough guess would be, that there is exactly one user using the port 1234 with Limesurvey.

There would be many users using the LS in a particular port because of the web environment. The number 1234 is just an example; I think you understood it;-)

I have reported it. Thanks for your help, Denis.

BTW: It is pity that there is no agency in my zone on the list in www.limesurvey.com/support

Well, today, and specially when it comes to IT and programming, physical location shouldn't be a problem, right? We here in the forum a spread out over the world as well and yet it works. Or is it a billing issue?

Yes, it would be best to pay to a local company by the school and we need an invoice by the local law and regulation. That's why I care for the location first.
I will pay I think to support our project somehow on my own. Maybe we could also be partners in the future.