What version of AEM are you running ?Can you clarify if the content fragment are placed into a template ?

Please check the following link where you can find good info about this task:Using Translation with AEM Content Fragments

Take the thread dumps (at least 5) as suggested during the time the issue happens and run analysis on it using thread dump analyzers available [1].[1] AEM Thread Dump Analysis  

- Make sure that XF is already published.- Make sure link externalizer is configured.- Verify if you can perform the export from the page editor, using comparable commands in the Page Information menu.

Then Its most likely caused by the known issue referenced by JaideepBrar above since you have the proper permissions and have been able to perform the same in older versions.

1. Can you clarify if you are using touch UI view or classic UI view ?2. Is this happening for custom components or OOTB components ?3. I am assuming you have been able to perform this task in the past so you have proper permissions over pages/contents (?)

I think you are referring to translation rules where you identify the content to translate for pages, components, and assets that are included in, or excluded from, translation projects.Identifying Content to Translate

Please check the following thread that might be helpful to you:Clientlibs not loading the fonts when placed in apps

Are you logged in to the instance as an admin ? If not please use admin to do so and verify the behavior. If still not able to perform the task enable the browser debug tool as it may give you some hint on whats the issue. The classic UI is not very good to handle the errors returned from the server...

Also this is a very good doc to understand Sling request processing:The Basics