Cannot open documents with onlyoffice from nextcloud, SSL certificate problem

[eauchat]

Describe the bug

Since I don't know exactly when, maybe an onlyoffice or nextcloud upgrade, documents don't open in nextcloud.
In onlyoffice forum someone was proposing to try deactivating the onlyoffice nextcloud plugin, reactivating it, and then saving config in nextcloud administration settings. I tried that, but when I try saving the config I get the following error:

Error when trying to connect (cURL error 60: SSL certificate problem: unable to get local issuer certificate (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://onlyoffice.domain.tld/healthcheck)

I thought maybe this could be an explanation why onlyoffice isn't working anymore. But I don't really understand what to do to resolve the issue.
Any idea?

Occasionally, doing the same operation shows a 502 gateway error message. Then it comes back to the message mentioned previously.
Indeed if I try to open https://onlyoffice.domain.tld/healthcheck it shows a 502 gateway error message.

Context

• Hardware: VM in a computer, nextcloud and onlyoffice are in different VMs
• YunoHost version: 11.2.3
• I have access to my server: Through SSH, through the webadmin
• Are you in a special context or did you perform some particular tweaking on your YunoHost instance?: see hardware situation
• Using package version/branch: 7.3.3~ynh1

[eauchat]

Today I tried reinstalling onlyoffice from scratch, hoping it would make a difference.
It's not working better, kind of worse in fact.
Nextcloud simply makes me download the document and doesn't even try anymore to open onlyoffice.
The https://onlyoffice.domain.tld/healthcheck page now displays the following message:
Cannot GET /healtcheck.