Have you checked permissions on the server? Under System Information in Joomla, you can check the permissions for common folders. If most appear in red color, then there is probably an ownership issue for files and folders and your hosting company should correct it.

OK, first things first. When you use the "Image Gallery" tab in K2, all the uploaded images are moved into /media/k2/galleries/ (in subfolders based on each item's ID). The reason this is done is similar to how images upload via the "Image" tab are moved into /media/k2/items/: keep the media out of the users reach so no accidents happen (e.g. deleted media). This kills re-usability, but experience has shown that content managers usually upload new images and most times keep a pool of media/images under /images in Joomla, if they want to re-use something.

Now, if you create "regular" image galleries using the "Simple Image Gallery Pro" editor button, then these image galleries will be stored under the "root path" that you've defined in SIGPro's component settings (the default is /images/).

I hope you see the distinction already between the 2 ways of inserting an image gallery in K2.

As for the errors you get, these are JS related and they could be caused by other plugins that can load in K2 pages (e.g. system or third-party K2 plugins).

Can you share 2 screenshots please? One with the form after you save the item and one with the browser dev tools in a separate window, again after the item is saved.

Try the 2.10 dev build uploaded just yesterday at: github.com/getk2/k2/archive/master.zip

A 500 error indicates a server issue. If the issue still occurs, it could be a third-party K2 plugin that is not up to date with PHP, K2 or Joomla.

Disable all 3rd party system plugins that do not belong to Joomla or K2 and then check again that page. Then, start enabling each plugin one by one until you find which one breaks the output.

Only modules are filtered for "site" and "administrator". You need to disable "Plugins" of "System" type.

First off, the Media Manager in K2 is a third-party solution integrated into K2.

Secondly, the original error indicated broken JS in the Media Manager. If the issue is resolved, I doubt it's the image size (you'd probably get a limit error if reached). One of the plugins that you disabled allowed JS to function properly in the Media Manager.

In any case, clear your browser's console, visit the Media Manager and attempt to upload a "larger" image that you said previously error-ed and see what the console outputs.