@mfavetti made a PR for limer that should fix this issue.
Using that PR I created a new fork of limer with that PR
You can test it by running See  github.com/cloudyr/limer/pull/66 and  github.com/Jan-E/limer

Thanks Jan, I confirm that this fork `"Jan-E/limer"` works fine (PHP 7.4 and limesurvey CE Version 6.3.6+231120 )

thanks Denis, for suggestion to turn on logging. I was on wrong track, but participating in discussion on bugtracker with mfavetti helped a lot.

Jane wrote:

See  github.com/cloudyr/limer/pull/66 and  github.com/Jan-E/limer

If the updated version force to update external tool : we need to reverse and do it in 7.X version according to semver
 

The update seemed to make a sense though, non standard response was turned into a standard one. In addition the fix in Limer (R) is told to be backwards compatible, so no issue here. However there are tools (languages) mentioned in the manual manual.limesurvey.org/RemoteControl_2_API#Example_and_helper

If there could be a quick check that those function after the update, that would be good, and also if a paragraph could mention this change in the same page, that might help.

R seems to be covered now.

r0bis wrote:

The update seemed to make a sense though, non standard response was turned into a standard one. In addition the fix in Limer (R) is told to be backwards compatible, so no issue here. However there are tools (languages) mentioned in the manual manual.limesurvey.org/RemoteControl_2_API#Example_and_helper

Make sense : sure at 100% !
Standard ? Header are not in jsonrpc v1 standard

r0bis wrote:

If there could be a quick check that those function after the update, that would be good, and also if a paragraph could mention this change in the same page, that might help.

Yes, but it still an API semver version broker.

You know why Firefox or Chrome have version number up to 100 : to respect external developer.

Limer neede to be updated as well for LS 5.6.45+, because the change to the application/json header has been backported. See the release notes github.com/LimeSurvey/LimeSurvey/commit/...73198a8dd42ef2cd550a