here is thunderbird debbug :

console.log: WebExtensions: Loading packed extension from C:\Program Files (x86)\Mozilla Thunderbird\extensions\sogo-connector@inverse.ca.xpi
console.log: WebExtensions: Loading packed extension from C:\Program Files (x86)\Mozilla Thunderbird\extensions\sendtocategory@jobisoft.de.xpi
console.log: WebExtensions: Loading packed extension from \server\AppData\Roaming\Thunderbird\Profiles\t7176kyp.default\extensions{e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi
console.log: WebExtensions: Loading add-on preferences from C:\Program Files (x86)\Mozilla Thunderbird\extensions\sendtocategory@jobisoft.de.xpi
console.log: WebExtensions: Firing profile-after-change listeners for sendtocategory@jobisoft.de
console.log: WebExtensions: Loading add-on preferences from C:\Program Files (x86)\Mozilla Thunderbird\extensions\sogo-connector@inverse.ca.xpi
console.log: WebExtensions: Firing profile-after-change listeners for sogo-connector@inverse.ca
console.log: WebExtensions: Loading add-on preferences from \server\AppData\Roaming\Thunderbird\Profiles\t7176kyp.default\extensions{e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi
console.log: WebExtensions: Firing profile-after-change listeners for {e2fda1a4-762b-4020-b5ad-a41df1933103}
[calBackendLoader] Using Thunderbird's builtin libical backend
calendar-overlay.js: failed to include 'chrome://sogo-connector/content/calendar/folder-handler.js'
SyntaxError: redeclaration of let _topmostWindow
folders-update.js: failed to include 'chrome://sogo-connector/content/calendar/folder-handler.js'
SyntaxError: redeclaration of let _topmostWindow
File: chrome://sogo-connector/content/calendar/folder-handler.js
Line: 1

Stack:

@chrome://sogo-connector/content/calendar/folder-handler.js:1:1
jsInclude@chrome://sogo-connector/content/messenger/folders-update.js:9:20
@chrome://sogo-connector/content/messenger/folders-update.js:19:1
jsInclude@chrome://sogo-connector/content/calendar/calendar-overlay.js:8:20
@chrome://sogo-connector/content/calendar/calendar-overlay.js:19:1
loadScript@resource:///modules/Overlays.jsm:561:31
load@resource:///modules/Overlays.jsm:226:33
load@resource:///modules/Overlays.jsm:48:14
registerNonBootstrapped/<@chrome://messenger/content/parent/ext-legacy.js:242:18
registerNonBootstrapped@chrome://messenger/content/parent/ext-legacy.js:236:21
folders-update.js: failed to include 'chrome://sogo-connector/content/calendar/default-classifications.js'
SyntaxError: redeclaration of let SICalendarDefaultClassifications
File: chrome://sogo-connector/content/calendar/default-classifications.js
Line: 1

Stack:

@chrome://sogo-connector/content/calendar/default-classifications.js:1:1
jsInclude@chrome://sogo-connector/content/messenger/folders-update.js:9:20
@chrome://sogo-connector/content/messenger/folders-update.js:19:1
jsInclude@chrome://sogo-connector/content/calendar/calendar-overlay.js:8:20
@chrome://sogo-connector/content/calendar/calendar-overlay.js:19:1
loadScript@resource:///modules/Overlays.jsm:561:31
load@resource:///modules/Overlays.jsm:226:33
load@resource:///modules/Overlays.jsm:48:14
registerNonBootstrapped/<@chrome://messenger/content/parent/ext-legacy.js:242:18
registerNonBootstrapped@chrome://messenger/content/parent/ext-legacy.js:236:21
folders-update.js: failed to include 'chrome://sogo-connector/content/messenger/mails-labels.js'
SyntaxError: redeclaration of let SIMailsLabels
File: chrome://sogo-connector/content/messenger/mails-labels.js
Line: 1

Stack:

@chrome://sogo-connector/content/messenger/mails-labels.js:1:1
jsInclude@chrome://sogo-connector/content/messenger/folders-update.js:9:20
@chrome://sogo-connector/content/messenger/folders-update.js:19:1
jsInclude@chrome://sogo-connector/content/calendar/calendar-overlay.js:8:20
@chrome://sogo-connector/content/calendar/calendar-overlay.js:19:1
loadScript@resource:///modules/Overlays.jsm:561:31
load@resource:///modules/Overlays.jsm:226:33
load@resource:///modules/Overlays.jsm:48:14
registerNonBootstrapped/<@chrome://messenger/content/parent/ext-legacy.js:242:18
registerNonBootstrapped@chrome://messenger/content/parent/ext-legacy.js:236:21
Starting SOGo Integrator code...
Custom startup code not available.
e: Error opening input stream (invalid filename?): chrome://sogo-connector/content/general/startup.js
no calendar available: checking extensions update right now.
number of handled extensions: 1
skipping startup, sogo-connector present
url: http://sogo-demo.inverse.ca/plugins/updates.php?plugin=sogo-connector@inverse.ca&amp;version=68.0.0&amp;platform=WINNT_x86-msvc
no data returned for 'sogo-connector@inverse.ca'
prepareRequiredExtensions - result: {"urls":[],"configuration":[],"uninstall":[]}
no available update for handled extensions
ABChecker type 1: Contacts
ContextManager getContext()
directoryChecker: _checkHTTPAvailability()
ContextManager getContext()
directoryChecker: _checkHTTPAvailability()
startup done
GOT STATUS: 404
GOT STATUS: 404
GOT STATUS: 404

Nextcloud synchronization is currently not supported by "SOGo Connector".
Only synchronization with SOGo backend works.

Christian,

Is it planned ?

Thanks.