There exists a bug(?) in Chrome version 83 which causes KC3's devtools panel to fail to load when you open the game.
It is easy to work around this by simply closing and reopening devtools, but you must remember to do this on every F5 as well, otherwise your KC3 will appear to be functional but will not update when you perform game actions.
This creates the potential for accidents, so more effective workarounds have been investigated.
An issue with Chrome 83's Site Isolation feature (intended to protect against i.e. Spectre) causes the chrome devtools network API to stop firing events when content from a different origin is loaded in an iframe.
Since KanColle itself is loaded in an iframe on DMM.com, this triggers the bug.
(see bug report at https://bugs.chromium.org/p/chromium/issues/detail?id=1081270 )
You have a few options at your disposal. You can either disable the feature causing the bug, or find a way to stay on Chrome v81.
The simplest fix is to disable Site Isolation. You can do this by navigating to about:flags/#site-isolation-trial-opt-out in your browser and setting the 'Disable site isolation' option to "Disabled (not recommended)".
Remember, Site Isolation is a security feature. Disabling it could leave you potentially vulnerable to certain types of attacks if you're not careful.
If you're already on a version earlier than 83, you can try disabling the auto-update mechanism. Unfortunately, Chrome doesn't make it easy to do so.
Of course, disabling updates comes with the caveat of not being able to receive new security features when they're published, so if you choose this route, you should only keep updates disabled until the bug has been reported as fixed.
No method is guaranteed, but the usual recommended method is to disable the GoogleUpdate.exe service. You can do this via taskmgr, msconfig, services.msc, or regedit, if you know what you're doing. (this method has not been verified)
You can also try using registry policies to disable updates, but this depends on Chrome actually listening to the policies in question. (this method has not been verified either)
A registry file to perform this step for you can be found here (right-click -> save link): https://raw.githubusercontent.com/planetarian/KCDocumentation/master/Chrome83KC3Fix/DisableChromeUpdates.reg
Alternatively, you can use an alternative version of Chrome that doesn't update automatically, such as Chromium.
This may be the most ideal option, as you can use this alternative browser purely for KC while your everyday browsing remains secure.
You can find a stable build of Chromium 81 in a .zip package here: https://www.googleapis.com/download/storage/v1/b/chromium-browser-snapshots/o/Win_x64%2F737173%2Fchrome-win.zip?generation=1580439619624464&alt=media
just extract it somewhere and run Chrome.exe.