When attaching a marriage event in the new Source Linker, I will always get an Error Toast that says "! Something went wrong. Try again later."
This happens on marriage events that I have added to a record. In the collection that I'm working on (Italian Birth Registrations), birth records often have marriage and death information added later in the margin. In the case of the marriage, adding the name of the spouse to the record, and then adding a marriage event, is the best way I've found to add this additional information to the tree with clear source information.
This used to work in the old source linker. I typically process several of these types of records every day. Once I switched to the new source linker, I get this error message. I can workaround this issue by adding the event to the couple directly in the person page and then coming back to source linker to complete the Attach. But then I'm typing in the marriage information twice which is both error prone and inefficient.
Here is a link to source linker for a case that I haven't done the workaround on. You can see that if you try to add the marriage information for Maria Driussi, when you click Attach, you'll see the error message.
https://www.familysearch.org/search/linker?pal=/ark:/61903/1:1:QVSW-X6SJ&id=GTYN-SJ7