Hi all,
first of all apologies if the same was already treated (although I did not find an exact reproduction)..Ok here it goes:

We are running LS (v. 3.25.17), on a VM, behind a reverse proxy.
I can access resources module to upload images, and some users also (until it stops workings for them). Curiously if, I ask these affected users to use the server name, instead of the reverse proxy virtual host name, it works again, so it seems that something in the proxy is not going too great with kcf (why I can and some users can' t after some time, beats me). The error/issue is a frozen screen when resource button is clicked.

https requests (service.com) >>> Reverse Proxy >>>> https LS (serverlm.com)   ;   (already forced to use https).

Any idea of what could be going on?

Thanks for the support in this.

Cheers
 

This is cleary a reverse proxy issue and needs to be fixed on the proxy level.

The error/issue is a frozen screen when resource button is clicked.

I recommend to update to a newer version of LS3.
Perhaps the issue is related to KCFinder caching routines.
github.com/LimeSurvey/LimeSurvey/commit/...a20ca268d71110990fe2
 

Well sorry the delay, but I have been collecting some more evidences:

It seems that going through a reverse proxy is not the only culprit, as some users bypassing the proxy also experience the same behavior.
What I did find is that in the log requests that bad behavior was associated with extra http requests:

Example of a good behavior request

XXXXXXX- - [31/Mar/2021:11:13:58 +0200] "GET /limesurvey/third_party/kcfinder/browse.php?language=%27es%27 HTTP/1.1" 200 4094
XXXXXXX- - [31/Mar/2021:11:13:58 +0200] "GET /limesurvey/third_party/kcfinder/js_localize.php?lng=en HTTP/1.1" 200 -
XXXXXXX - - [31/Mar/2021:11:13:58 +0200] "GET /limesurvey/index.php/admin/survey/sa/getAjaxQuestionGroupArray/surveyid/982748?YII_CSRF_TOKEN=bHpFbHNrRUo3OVdldmtHdVF1NzFoU0ZfWkFfNzNUVlMWacOSpkLBmt5Ehss8V3slx1jFD3O3sxMmfjX8hTJ4RA%3D%3D HTTP/1.1" 200 4623
XXXXXXX- - [31/Mar/2021:11:13:58 +0200] "GET /limesurvey/index.php/admin/survey/sa/getAjaxMenuArray/surveyid/982748?YII_CSRF_TOKEN=bHpFbHNrRUo3OVdldmtHdVF1NzFoU0ZfWkFfNzNUVlMWacOSpkLBmt5Ehss8V3slx1jFD3O3sxMmfjX8hTJ4RA%3D%3D&position=side HTTP/1.1" 200 38399
XXXXXXX - - [31/Mar/2021:11:13:58 +0200] "GET /limesurvey/third_party/kcfinder/browse.php?type=files&lng=en&act=init&kcfinder_csrftoken=ed5ce92a18401df97e642baa1f7f2a02cb524acc2090ab6bcfd19e53474b86870b69b598c09911a6effb0ee4f0df762812bcbfbd80ba2a9ae493559b16d60d0a HTTP/1.1" 200 695
XXXXXXX - - [31/Mar/2021:11:13:58 +0200] "GET /limesurvey/index.php/admin/survey/sa/getAjaxMenuArray/surveyid/982748?YII_CSRF_TOKEN=bHpFbHNrRUo3OVdldmtHdVF1NzFoU0ZfWkFfNzNUVlMWacOSpkLBmt5Ehss8V3slx1jFD3O3sxMmfjX8hTJ4RA%3D%3D&position=collapsed HTTP/1.1" 200 19768

Example of a bad request:

XXXXXXXX- - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/third_party/kcfinder/browse.php?language=%27es%27 HTTP/1.1" 200 4094
XXXXXXXX- - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/third_party/kcfinder/css/index.php HTTP/1.1" 304 -
XXXXXXXX- - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/third_party/kcfinder/js_localize.php?lng=en HTTP/1.1" 200 -
XXXXXXXX- - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/third_party/kcfinder/themes/default/js.php HTTP/1.1" 304 -
XXXXXXXX- - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/third_party/kcfinder/themes/default/css.php HTTP/1.1" 304 -
XXXXXXXX- - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/third_party/kcfinder/js/index.php HTTP/1.1" 304 -
XXXXXXXX- - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/index.php/admin/survey/sa/getAjaxMenuArray/surveyid/982748?YII_CSRF_TOKEN=cWdkT05Xc2ltN0gyc1I2aUhMNGR2enNZeUxVTnVsUTLtLNkRRYXRm5YtQTPmgWMcHV1q8AkDv5jK0Gl64bsT3Q%3D%3D&position=collapsed HTTP/1.1" 200 21516
XXXXXXXX - - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/index.php/admin/survey/sa/getAjaxQuestionGroupArray/surveyid/982748?YII_CSRF_TOKEN=cWdkT05Xc2ltN0gyc1I2aUhMNGR2enNZeUxVTnVsUTLtLNkRRYXRm5YtQTPmgWMcHV1q8AkDv5jK0Gl64bsT3Q%3D%3D HTTP/1.1" 200 4632
XXXXXXXX - - [31/Mar/2021:11:24:55 +0200] "GET /limesurvey/index.php/admin/survey/sa/getAjaxMenuArray/surveyid/982748?YII_CSRF_TOKEN=cWdkT05Xc2ltN0gyc1I2aUhMNGR2enNZeUxVTnVsUTLtLNkRRYXRm5YtQTPmgWMcHV1q8AkDv5jK0Gl64bsT3Q%3D%3D&position=side HTTP/1.1" 200 38399

Notice the 304 code for that resources in bold.
On the console of a user that experiences the freeze, the console shows that it hangs on the call

admincore confirmationmodal calling adminbasics.js

I already asked the users to delete cache, or run Edge without extensions, without success so far.
And this is what I have at the moment. Any ideas?

Thanks