IMAP connection problem

Hello, I am having trouble connecting to my IMAP server (the SMTP server is working fine). Any help appreciated!

I use Dovecot on port 993, and I checked that it is working with TLS:

openssl s_client -connect myserver.com:993
CONNECTED(00000003)
depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
verify return:1
depth=0 CN = myserver.com
verify return:1
---
Certificate chain
0 s:/CN=myserver.com
i:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
1 s:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
i:/O=Digital Signature Trust Co./CN=DST Root CA X3
---
Server certificate
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----
subject=/CN=myserver.com
issuer=/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: ECDH, P-384, 384 bits
---
SSL handshake has read 3177 bytes and written 334 bytes
Verification: OK
---
New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol : TLSv1.2
Cipher : ECDHE-RSA-AES128-GCM-SHA256
Session-ID: CD02D04A17D9708940E0EBF7CEA8179B4475693305DC789B290280984287841C
Session-ID-ctx: 
Master-Key: 80F37A6B5233A1386398645BAD916EE53DFDF6D74F3D4A81642A289146A7A4956601DB6492FB902AE97B10D04E98EEB2
PSK identity: None
PSK identity hint: None
SRP username: None
TLS session ticket lifetime hint: 7200 (seconds)
TLS session ticket:
...

Start Time: 1523482600
Timeout : 7200 (sec)
Verify return code: 0 (ok)
Extended master secret: yes
---
* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE AUTH=PLAIN AUTH=LOGIN] Dovecot ready.

I have also double-checked my login credentials by logging in to webmail using the same username and password.

On the “mailbox settings” page, I selected:
Port: 993
Protocol: IMAP
Secure Setting: TLS
Use Secure Authentication: Yes

Here is the diagnostic output from the plugin’s main page:

System Properties		 
WP Mailster Free: 	1.5.8 BUILD 116 	 
Date: 	March 2018 	 
Operating System: 	Linux myhost 4.15.7-x86_64-linode102 #1 SMP Wed Feb 28 16:39:31 UTC 2018 x86_64 	 
PHP version: 	7.0.27-0+deb9u1 	 
Max. Execution Time: 	30 	 
Memory Limit: 	256M (10 MB) 	 
parse_ini_file: 	OK 	 
PHP Disabled Functions: 	, pcntl_alarm, pcntl_exec, pcntl_fork, pcntl_get_last_error, pcntl_getpriority, pcntl_setpriority, pcntl_signal, pcntl_signal_dispatch, pcntl_sigprocmask, pcntl_sigtimedwait, pcntl_sigwaitinfo, pcntl_strerror, pcntl_wait, pcntl_waitpid, pcntl_wexitstatus, pcntl_wifcontinued, pcntl_wifexited, pcntl_wifsignaled, pcntl_wifstopped, pcntl_wstopsig, pcntl_wtermsig 	 
Database version: 	5.5.5 	 
	imap_mime_header_decode (Test OK)
	 
WordPress version: 	4.9.5 	 
Site Url: 	https://www.mysite.com 	 
IMAP: 	Loaded 	 
  	Open mailbox timeout: 20 (Global setting: 60) 	 
  	

Array
(
    [IMAP c-Client Version] => 2007f
    [SSL Support] => enabled
    [Kerberos Support] => enabled
)

	 
OpenSSL: 	Loaded 	 
  	

Array
(
    [OpenSSL support] => enabled
    [OpenSSL Library Version] => OpenSSL 1.1.0f 25 May 2017
    [OpenSSL Header Version] => OpenSSL 1.1.0f 25 May 2017
    [Openssl default config] => /usr/lib/ssl/openssl.cnf
    [openssl.cafile] => no value
    [openssl.capath] => no value