This error usually indicates that SQL Server is very busy and/or has insufficient resources available to execute the queries used by Activity Monitor. I would recommend checking SQL manually using a query such as "sp_who2" to see what is actively running. You may also want to look at Task Manager or Resource Monitor on the server itself. I've also included a query below that you can execute in SQL Server to get an understanding of CPU utilization. Take care.
DECLARE @ts_now BIGINT
select @ts_now = cpu_ticks / (cpu_ticks/ms_ticks) from sys.dm_os_sys_info;
DATEADD(ms, -1 * (@ts_now - [timestamp]), GETDATE()) AS EventTime,
100 - SystemIdle - SQLProcessUtilization AS OtherProcessUtilization
--CAST(record AS XML)
record.value('(./Record/@id)', 'bigint') AS record_id,
record.value('(./Record/SchedulerMonitorEvent/SystemHealth/SystemIdle)', 'bigint') AS SystemIdle,
record.value('(./Record/SchedulerMonitorEvent/SystemHealth/ProcessUtilization)', 'bigint') AS SQLProcessUtilization,
SELECT TIMESTAMP, CONVERT(XML, record) AS record
WHERE ring_buffer_type = N'RING_BUFFER_SCHEDULER_MONITOR'
AND record LIKE '% %') AS x
) AS y
ORDER BY record_id DESC