Any JavaScript error in your console?

Which IMAP server are you using?

If it's Dovecot, you might have to increase your imap_max_line_length

No Javascript error in console!
increased imap_max_line_length to 1M didn't help

some of Dovecot settings:

auth_worker_max_count = 30
imap_max_line_length = 1 M
imapc_max_idle_time = 29 mins
imapc_max_line_length = 0
login_proxy_max_disconnect_delay = 0
mail_max_keyword_length = 50
mail_max_lock_timeout = 0
mail_max_userip_connections = 10
mail_sort_max_read_count = 0
managesieve_max_compile_errors = 5
managesieve_max_line_length = 65536
autoexpunge_max_mails = 0
replication_max_conns = 10
mail_max_userip_connections = 30

On Outlook it works just fine !! The Problem only happens on the web mail!

Folders with more than 5000 messages don't even show up! Outlook works fine so does not seem to be a Dovecot problem!

I'm having the same problem, using Chrome browser. Nothing shows up, except status messages like "Waiting for server.com". No JavaScript errors.

/var/log/dovecot/current gets filled with messages like these:

2017-07-06 21:44:19.917589500 imap(holck): Info: Disconnected: Logged out bytes=132/2057
2017-07-06 21:44:19.971815500 imap-login: Info: Login: user=<holck>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=27072, secured
2017-07-06 21:44:20.220712500 imap(holck): Info: Disconnected: Logged out bytes=132/2057
2017-07-06 21:44:20.281480500 imap-login: Info: Login: user=<holck>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=27073, secured
2017-07-06 21:44:20.529720500 imap(holck): Info: Disconnected: Logged out bytes=132/2057
2017-07-06 21:44:20.926715500 imap-login: Info: Login: user=<holck>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=27074, secured
2017-07-06 21:44:21.175706500 imap(holck): Info: Disconnected: Logged out bytes=132/2057
2017-07-06 21:44:21.928724500 imap-login: Info: Login: user=<holck>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=27075, secured
2017-07-06 21:44:22.177703500 imap(holck): Info: Disconnected: Logged out bytes=132/2057
2017-07-06 21:44:22.936908500 imap-login: Info: Login: user=<holck>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=27076, secured
2017-07-06 21:44:23.185720500 imap(holck): Info: Disconnected: Logged out bytes=132/2057
2017-07-06 21:44:23.943383500 imap-login: Info: Login: user=<holck>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=27077, secured
2017-07-06 21:44:24.190756500 imap(holck): Info: Disconnected: Logged out bytes=132/2057

/var/log/sogo/sogo.log shows:
Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0
Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/0/view HTTP/1.1" 200 4420/0 0.235 - - 16K
Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0
Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/0/view HTTP/1.1" 200 4420/0 0.234 - - 16K
Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0
Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/0/view HTTP/1.1" 200 4420/0 0.236 - - 16K
Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0
Jul 06 21:44:20 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/0/view HTTP/1.1" 200 4420/0 0.236 - - 12K
Jul 06 21:44:20 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0
Jul 06 21:44:20 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/0/view HTTP/1.1" 200 4420/0 0.236 - - 16K
Jul 06 21:44:20 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0
Jul 06 21:44:21 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/0/view HTTP/1.1" 200 4420/0 0.236 - - 16K

Dovecot documentation states that errors like "Too long argument" or "IMAP command line too large" should appear in log but they don't !!

We increased imap_max_line_length to 15M and still have same result !

Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0

This means, that it can not fetch its email display frame.
That is your problem.

Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/0/view HTTP/1.1" 200 4420/0 0.235 - - 16K

That means, that the list of emails in that INBOX is read successfully.
Therefore no dovecot problem.

What can you see from the log below?
doesn't give any clue right?

Jul 11 08:07:46 sogod [53141]: mx.myaddress.com "GET /SOGo/so/leyla@myaddress.com/Mail/view HTTP/1.0" 200 17304/0 0.058 73691 76% 0
Jul 11 08:07:46 sogod [53141]: mx.myaddress.com "GET /SOGo/so/leyla@myaddress.com/Calendar/alarmslist?browserTime=1499774865 HTTP/1.0" 200 63/0 0.007 - - 0
Jul 11 08:07:47 sogod [53141]: mx.myaddress.com "GET /SOGo/so/leyla@myaddress.com/Mail/0/view HTTP/1.0" 200 5477/0 0.292 - - 0

No errors here.
Do you get errors in your reverse proxy (Apache or nginx) for those?

No errors on nginx.log

Jul 06 21:44:19 sogod [27038]: 192.168.10.238 "GET /SOGo/so/holck/Mail/UIxMailMainFrame HTTP/1.1" 404 208/0 0.007 - - 0

This means, that it can not fetch its email display frame.
That is your problem.

Is there anything I can do to overcome this problem?

No errors here.
Do you get errors in your reverse proxy (Apache or nginx) for those?

I'm running Apache and getting lots of lines like these in /var/log/httpd/error.log:
[Thu Jul 06 21:45:39 2017] [error] proxy: HTTP: disabled connection for (127.0.0.1)
[Thu Jul 06 21:45:39 2017] [error] proxy: HTTP: disabled connection for (127.0.0.1)

Sogo works fine in version 3.1.4

Update:

I removed an old part of my Apache-configuration file, httpd.conf, relating to proxy.pac, and now sogo 3.2.10 runs nicely. Sorry for the confusion, thanks for your help.