DataGuard monitoring is generating errors (4341996)

An unexpected problem has occurred
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Details:
Message:
Access violation at address 1666B2E1 in module 'DataGuard.spx'. Read of address 00000000

Source:
1666B2E1 (DataGuard.0026A2E1) 8.1.0.1654

Call stack:
1666B2E1 (DataGuard.0026A2E1) 8.1.0.1654
16404D12 (DataGuard.00003D12) 8.1.0.1654
7C90327A (ntdll.0000227A) 5.1.2600.6055
7C90E48A (ntdll.0000D48A) 5.1.2600.6055
1666AD2A (DataGuard.00269D2A) 8.1.0.1654
005AF8C4 (Spotlight.001AE8C4)
0082CD58 (Spotlight.0042BD58)
00824AE7 (Spotlight.00423AE7)
008306F3 (Spotlight.0042F6F3)
004C9E71 (Spotlight.000C8E71)
004CE378 (Spotlight.000CD378)
004AA2E9 (Spotlight.000A92E9)
004CDA8F (Spotlight.000CCA8F)
00435E96 (Spotlight.00034E96)
7E418734 (USER32.00007734) 5.1.2600.5512
7E418816 (USER32.00007816) 5.1.2600.5512
7E4189CD (USER32.000079CD) 5.1.2600.5512
7E418A10 (USER32.00007A10) 5.1.2600.5512
004B2A2A (Spotlight.000B1A2A)

Datasource type: DatasourceSQLNet
Handler: TableResultsUpdated
Table: ActiveInstances
Exception:
Event handler already registered (IDatasourceActivity.RegisterEventHandler)
Source: Source:
16636D97 (DataGuard.00235D97) 8.1.0.1654
Call stack:
16636D97 (DataGuard.00235D97) 8.1.0.1654
16648318 (DataGuard.00247318) 8.1.0.1654
166F8A0F (DataGuard.002F7A0F) 8.1.0.1654
166F978B (DataGuard.002F878B) 8.1.0.1654
166AD616 (DataGuard.002AC616) 8.1.0.1654
1650AD4B (DataGuard.00109D4B) 8.1.0.1654
16634659 (DataGuard.00233659) 8.1.0.1654