APM: Problems with FCKEditor in Chrome or Edge

I'm deploying an APM on a customer that has several portal links. It's all working as it should except for a custom application that insists on not working when using Google Chrome or Microsoft Edge. The application has an editor page with FCKEditor that opens and editor and I can only see it with Firefox. I've checked the browser Developer Tools and the errors are different on Chrome or Firefox although Firefox shows it.

Here's the Firefox log:

Uncaught TypeError: F5_Deflate_value(...) is undefined
    ProcessHiddenField https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:33
    <anonymous> https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/fckeditor.html?InstanceName=UpdateContent&Toolbar=Default:166
fckeditorcode_gecko.js:33:1859


Uncaught TypeError: A is null
    ResetStyles https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:36
    FCKPanel https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:85
    FCKContextMenu https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:104
    FCK_ContextMenu_Init https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:105
    <anonymous> https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/fckeditor.html?InstanceName=UpdateContent&Toolbar=Default:207
fckeditorcode_gecko.js:36:11619


Uncaught TypeError: A is null
    ResetStyles https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:36
    FCKPanel https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:85
    FCKSpecialCombo https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:89
    Create https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:90
    Create https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:97
    Load https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J:99
    LoadToolbar https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/fckeditor.html?InstanceName=UpdateContent&Toolbar=Default:244
    StartNext https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J line 38 > eval:1
    Add https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/js/fckeditorcode_gecko.js?F5CH=J line 38 > eval:1
    LoadToolbarSetup https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/fckeditor.html?InstanceName=UpdateContent&Toolbar=Default:232
    <anonymous> https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/fckeditor.html?InstanceName=UpdateContent&Toolbar=Default:227
    onload https://XXX.XXX.XXX.XXX/private/fm/cache-fm.js:1025
    F5_Inflate_onload https://XXX.XXX.XXX.XXX/private/fm/cache-fm.js:1023
    <anonymous> https://XXX.XXX.XXX.XXX/f5-w-687474703a2f2f7370632d3031612e6c6a632e6c6f63616c$$/NoAuth/RichText/editor/fckeditor.html?InstanceName=UpdateContent&Toolbar=Default:220
fckeditorcode_gecko.js:36:11619

And here's the Chrome log:

fckeditorcode_gecko.js?F5CH=J:33 Uncaught TypeError: Cannot read property 'split' of undefined
    at Object.FCKConfig.ProcessHiddenField (fckeditorcode_gecko.js?F5CH=J:33)
    at VM360 fckeditor.html:166
FCKConfig.ProcessHiddenField @ fckeditorcode_gecko.js?F5CH=J:33
(anonymous) @ VM360 fckeditor.html:166


fckeditorcode_gecko.js?F5CH=J:36 Uncaught TypeError: Cannot read property 'style' of null
    at Object.FCKTools.ResetStyles (fckeditorcode_gecko.js?F5CH=J:36)
    at new FCKPanel (fckeditorcode_gecko.js?F5CH=J:85)
    at new FCKContextMenu (fckeditorcode_gecko.js?F5CH=J:104)
    at FCK_ContextMenu_Init (fckeditorcode_gecko.js?F5CH=J:105)
    at VM371 fckeditor.html:207
FCKTools.ResetStyles @ fckeditorcode_gecko.js?F5CH=J:36
FCKPanel @ fckeditorcode_gecko.js?F5CH=J:85
FCKContextMenu @ fckeditorcode_gecko.js?F5CH=J:104
FCK_ContextMenu_Init @ fckeditorcode_gecko.js?F5CH=J:105
(anonymous) @ VM371 fckeditor.html:207


cache-fm.js:628 Uncaught TypeError: Cannot read property 'appendChild' of null
    at F5_CallEmbed (cache-fm.js:628)
    at F5_Invoke_appendChild (cache-fm.js:664)
    at InitializeAPI (fckeditorcode_gecko.js?F5CH=J:38)
    at VM377 fckeditor.html:222
    at F5_Inflate_onload.o.onload (cache-fm.js:1025)
F5_CallEmbed @ cache-fm.js:628
F5_Invoke_appendChild @ cache-fm.js:664
InitializeAPI @ fckeditorcode_gecko.js?F5CH=J:38
(anonymous) @ VM377 fckeditor.html:222


F5_Inflate_onload.o.onload @ cache-fm.js:1025
load (async)
F5_Inflate_onload @ cache-fm.js:1023
(anonymous) @ VM377 fckeditor.html:220

Anyone got a similar issue that can shed some lights on the matter or have any suggestion on what/how to debug more deeply? I cannot access the server or code since it's a closed customer application but I have full F5 access.