I'm getting a peculiar error, something I haven't seen before, when I run test classes. All the failing tests were passing successfully earlier (a week ago).
System.DmlException: Insert failed. First exception on row 0; first error: UNABLE_TO_LOCK_ROW, unable to obtain exclusive access to this record or 1 records: 07H7Y000000Cc1RUAS: 
The 07H prefix refers to ContentFolder.
I get this error on some test classes where I refer to some Content- object. If I run these tests individually, some of them pass while some won't even pass if run them on their own.
I have one Library, which is the standard
Assets Library. The error happens when I call
insert on a list of ContentVersion records. I'm running these tests as System Administrator on our sandbox org.
How would I go about debugging this error?
Thanks in advance.