Lync cannot connect to the exchange server EWS

It has been a week now that some of the users in my company are asking for login credentials after they login to the lync 2010 client. Please see the screenshot below:

When I cancel the option by pressing the cancel button, lync functions properly but at the same time on the bottom right corner I get an error message. The message says:

Lync cannot connect to the exchange server

After this I changed the connection settings to manual configuration on both the internal and external servers. I checked the configuration information by pressing ctrl + right click, the menu opened.  The MAPI status was OK but the EWS was not available. The configuration settings are as follows:

DG URL Internal;https://webpoolsn20a02.infra.lync.com:9999/groupexpansion/service.svc;–;
DG URL External;https://webpoolsn20a02.infra.lync.com:443/groupexpansion/service.svc;–;
Quality Metrics URI;sip:[email protected];gruu;opaque=srvr:HomeServer:*********************;–;
URL Internal From Server;https://webpoolsn20a02.infra.lync.com:9999/abs/handler;–;
URL External From Server;https://webpoolsn20a02.infra.lync.com:443/abs/handler;–;
Voice mail URI;sip:********@******;opaque=app:voicemail;–;
Exum Enabled;FALSE;–;
Exum URL;;–;
MRAS Server;sip:[email protected];gruu;opaque=srvr:MRAS:***************************;Enabled;
GAL Status;https://webpoolsn20a02.infra.lync.com:9999/abs/handler;–;
Controlled Phones;TRUE;–;
PC to PC AV Encryption;AV Encryption Supported;–;
Focus Factory;sip:[email protected];gruu;opaque=app:conf:focusfactory;–;
Telephony Mode;Telephony Mode Disabled;–;
Line;;–;
Line Configured From;Auto Line Configuration;–;
Location Profile;DefaultProfile;–;
Call Park Server URI;;–;
UCS Mode;Lync Server Mode;–;
Configuration Mode;Manual Configuration;–;
Server Address Internal;sipdir.online.lync.com:443;–;
Server Address External;sipdir.online.lync.com:443;–;
Server SIP URI;*********@********;–;
GAL or Server Based Search;Server Based Search;–;
Local Log Folder;C:UsersGabortracing;–;
MAPI Information;MAPI Status OK;MAPI Status OK;
EWS Information;–;EWS unavailable;
Inside User Status;FALSE;–;
Auto Update Download Started;–;–;
Auto Update Download Completed;–;–;
Last Auto Update Request;–;–;
Pairing State;Lync cannot connect to your desk phone because the USB cable is not plugged in. Make sure that you connect the cable.;Enabled;
Contact List Provider;Lync Server;–;
UCS Connectivity State;Exchange connection Down;–;
Connected Lync Server;sippoolsn20a02.infra.lync.com;–;
Skill Search URL;;–;
SharePoint Search Center URL;;–;
EWS Internal URL;;–;
EWS External URL;;–;
Server SIP URI;*********@***********;TLS Mode;

The only solution I could think of was to reinstall Lync but unfortunately even this didn’t help. If anyone has any idea how to fix the issue, I'll be really thankful if you share it with me.

Waiting for your replies

Regards