I'm posting this because I could not find a useful reply to others who have asked about this error. I just encountered it for the first time today. I presume there are other reasons for this error, I'm just offering the one that affected me today:
My broker explained something I had not known before. I had thought of the forex market as 24/5. But there is a gap regarding transactions for 5-10 minutes each day beginning at 2100GMT when they process their rollovers. So if you want to enter or modify or exit your trade during this period, you're out of luck.
Now a second chat agent says it lasts a matter of seconds, took 47 seconds today.
Hope this might help someone. Did you get the error around 21:00 GMT?