In my ULS Log is frequently the following entry:
Unexpected: CannotMakeBrowserCacheSafeLayoutsUrl: 15/1033/styles/core.css
What does it mean?
How can I fix it?
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It looks like you are calling CSSRegistration on a css file that doesn't exist. CSSRegistration is used to ensure that a css file will only be loaded once even if it is referenced multiple times. It can also allow for conditional loading of stylesheets depending on what browser is being used to view the page.
Have a look through any masterpages or page layouts you are using for a line similar to the following
<SharePoint:CSSRegistration ID="CSSRegistration0" name="15/1033/styles/core.css" runat="server" />
If you are actually intending to reference the Core css file then the path is "_layouts/15/1033/styles/corev15.css" in SharePoint 2013. To call CSSRegistration on this file, use the following in you master page.
<SharePoint:CSSRegistration ID="CSSRegistration0" name="_layouts/15/1033/styles/corev15.css" runat="server" />
Here is a link with a bit more info on CSSRegistration. http://tommdaly.wordpress.com/2012/05/02/sharepoint-cssregistration-or-link/
I proposed an answer here https://social.msdn.microsoft.com/Forums/office/en-US/a5f69d88-5caf-4a82-a720-e7bc8d01b7c8/cannotmakebrowsercachesafelayoutsurl-151033stylesdiscthreadcss-error-in-uls-logs?forum=sharepointadmin
We experienced slightly different error messages:
CannotMakeBrowserCacheSafeLayoutsUrl: 15/1033/styles/calendar.css CannotMakeBrowserCacheSafeLayoutsUrl: 15/1033/styles/core.css
This may be related to a Nintex upgrade https://support.nintex.com/SharePoint/Forms/2013/Error_%22Cannot_make_a_cache_safe_URL...%22_after_upgrading
It was hard to debug the error. The path '15/1033/styles/core.css did not display in the source of masterpages or other pages. I had to add a web part to a Nintex Form page and attach a debugger and step through loading a page. Intellitrace showed this message.
In addition to deploying nintexforms2013backwardscompatibilityui.wsp to the farm, I also copied core.css to the path listed in the error.