EndNote X5 was released in 2011, and there are no known compatibility issues with Word 2010. The most likely cause of your problems is the Instant Formatting feature.
When Instant Formatting is on, each time you insert a citation into a document EndNote has to update the entire document. If you have a large document with a large number of citations, this can cause some slowness when you are inserting citations.
Likewise, if Instant Formatting is on and is set to scan the document for temporary citations periodically and to check for citation changes, you will most likely experience a crash if you try to copy and paste text that contains formatted citations. If you are trying to copy and paste individual citations, you should convert the citations to unformatted citations first. Formatted citations contain field codes, and the field codes can become damaged when individual citations are copied and pasted.
To unformat an individual citation, highlight the citation, and on the EndNote X5 tab click on Convert Citations and Bibliography > Convert to Unformatted Citations.
If you are copying and pasting blocks of text that contain formatted citations you should either turn off Instant Formatting or at least turn off the background scanning.
When you have the document open in Word go to the EndNote X5 tab. You will notice that the tab is divided into three sections. The middle section is called Bibliography, on the right end of the bar that says Bibliography is an icon that looks like a square with a diagonal arrow.
Click on that icon to open the Bibliography Settings window. Click on the Instant Formatting tab.
If you want to turn off Instant Formatting for the document, click on Turn Off and then click on Cancel.
If you simply want to turn off background scanning click on the Settings button and uncheck the boxes for “Scan for temporary citations” and “Check for citation changes.”
Finally if you are tracking changes in a document, before you use any of the EndNote commands in Word, go to the Review tab in Word and in the tracking window change “FInal Showing Markup” to “Final.” This will hide the tracked changes. If the tracked changes are visible when you use the EndNote commands you may experience document corruption and receive any number of error messages.