JSON ERROR on save with Current Language Link option enabled

[leotiger]

Describe the bug
Once I activate the option Current Language Link I receive in all language versions (subdirectory) the following json error while saving a page: Updating failed. The response is not a valid JSON response.

To Reproduce
Steps to reproduce the behavior:

1. Go to Plugin Settings
2. Activate Current Language Link
3. Open a Page in Editor (I'm using FSE, only essential plugins, cache plugin and other optimizing plugins disabled)
4. Save the page and see the json error

Expected behaviour
Possibility to use the feature

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: MacOS
• Browser Firefox
• PHP Version [e.g. 7.2]
• Plugin Version [e.g. 2.2.0]

Additional context
Obviously this error may depend on my server configuration (apache+nginx). Nevertheless, once the option is disabled, the rest works fine... Probably I'll dig a bit deeper into this. I've been searching through closed and open tickets before supplying this report.

[leotiger]

Found the problem. With debug there was a deprecation message informing to use:

'''
if ( function_exists( 'msls_the_switcher' ) ) msls_the_switcher();
'''

Once I deactivated this in my theme function file, things worked. Wasn't even the Current Language Link option, the plugin simply failed to work once activated and configured with any option on more than one multisite.

And the proposed function in the documentation does not work for me, either:

'''
if ( function_exists( 'the_msls' ) ) the_msls();
'''

Not sure why. May still be a local problem on my side.

[lloc]

Is it the latest MSLS version (2.10.1) creating this problem in your setup?

[leotiger]

Yeah, latest version. I'll do some further testing now as I need the language switcher capabilities the plugin offers.

In fact, I don't need the script implementation as I can place the switcher as a block (hopefully) and if it works, great. Will keep you updated. I encountered another problem with import for pages that convert into posts on import. I will open up a dedicated ticket if I can confirm this.