Reckon Accounts 2018 Restarts Intermittently; QBWin and QBSession Logs shows multiple DB errors

My Client uses Reckon Accounts 2018 - and they prepare & send invoices from a 'client' desktop PC that different & separate to the 'host' desktop PC that stores the Company File and has Reckon Accounts Database Server installed.
However, they have been experiencing an intermittent issue for the past 2 months where - at a random time and day - Reckon will display an error message apologising that it has to restart as they attempt to save an invoice, or while they're still working on creating an invoice.

Looking at the QBWin and QBSession logs, the following error messages typically appear when Reckon forces a restart - where these are the relevant lines from the QBSession log:

dbspecificerrorhandler.cpp (70) :  MESSAGE: Wed Aug 29 11:32:15 LVL_ERROR--DB error -298 ErrorMessage:'Attempted two active database requests' from file:'.\.\src\DMEDLSQLItemEditListElement.cpp' at line 1152 from function:'DMEDLItemEditListElement::DBLoad'
dbspecificerrorhandler.cpp (70) :  MESSAGE: Wed Aug 29 11:32:15 LVL_ERROR--DB error -298 ErrorMessage:'Attempted two active database requests' from file:'.\.\src\DMEDLSQLItemEditListElement.cpp' at line 1485 from function:'DMEDLItemEditListElement::DBLoad'
CEDLIST.c (8594) :  MESSAGE: Wed Aug 29 11:32:15 LVL_INFO_DETAIL--Error on edlist factory : Type 3 Error: -6024;A record could not be loaded from a edlist table.
DMDQEDynamicQuery.cpp (1340) :  MESSAGE: Wed Aug 29 11:32:16 LVL_SEVERE_ERROR--ExecuteQuery failed (DETAIL AVAILABLE), error id: -6019, sub id: 0, 'Succeeded', ''
DMDQEDynamicQuery.cpp (1346) :  MESSAGE: Wed Aug 29 11:32:16 LVL_INFO_DETAIL--Query: SELECT abmc_item_assembly_level_graph.level , abmc_bill_lineitem.target_id , abmc_bill_lineitem.transaction_id , abmc_bill_lineitem.transaction_view_type ,  CAST (( IF abmc_bill_lineitem.is_groupend_row_bool = 1 THEN 2147482648 ELSE abmc_bill_lineitem.item_id ENDIF ) AS  INTEGER ) AS item_id , abmc_bill_lineitem.transaction_date , abmc_bill_inventory_lineitem.account_id , abmc_bill_inventory_lineitem.amount_amt , null, null, abmc_bill_inventory_lineitem.quantity_qnty , abmc_bill_lineitem.is_source_bool , abmc_bill_lineitem.is_no_post_bool , null, abmc_bill_lineitem.is_purchase_bool , null, null, abmc_bill_lineitem.is_receipt_bool , null, abmc_bill_lineitem.is_received_bool , null, null, null, null, abmc_bill_lineitem.is_manually_closed_bool , abmc_bill_lineitem.is_invoiced_bool , abmc_bill_lineitem.open_qnty , abmc_bill_lineitem.unit_price_amt , abmc_bill_lineitem.unit_price_qnty , abmc_bill_lineitem.unit_price_pct , null, null, null, abmc_bill_inventory_lineitem.inventory_record_id , abmc_bill_inventory_lineitem.target_type 
 FROM abmc_item_assembly_level_graph, abmc_bill_lineitem, abmc_bill_inventory_lineitem, abmc_item_assembly_level_graph
 WHERE (((abmc_bill_lineitem.transaction_date  >=  abmc_item_assembly_level_graph.transaction_date) AND (abmc_item_assembly_level_graph.level  >= '0')) AND ((abmc_bill_lineitem.item_id  IS NOT NULL) AND (abmc_bill_lineitem.item_id  != '0'))) AND abmc_bill_lineitem.item_id = abmc_item_assembly_level_graph.item_id AND (abmc_bill_inventory_lineitem.target_id = abmc_bill_lineitem.target_id) AND (abmc_bill_lineitem.is_memorized_transaction_bool = '0')
DQEWrapper.cpp (422) :  MESSAGE: Wed Aug 29 11:32:16 LVL_SEVERE_ERROR--DQEWrapper::DoQuery: ExecuteQuery failed, error id: -6019, sub id: 0, 'Succeeded'
ItemHistUpdateEngine.cpp (728) :  MESSAGE: Wed Aug 29 11:32:16 LVL_ERROR--Item History database error (6).  Error Message: Succeeded
dbspecificerrorhandler.cpp (66) :  MESSAGE: Wed Aug 29 11:32:16 LVL_WARNING--DB error -170 ErrorMessage:'Cursor has not been declared' from file:'.\.\src\SQLDynamicQuery.cpp' at line 540 from function:'DQE::DMDQEDynamicQuery::DBSQLCloseCursor'
master.c (13095) :  MESSAGE: Wed Aug 29 11:32:16 LVL_WARNING--Could not load Invoice Batch Header master for recnum 1321392.  Record not found.  Perhaps it was deleted?
dbspecificerrorhandler.cpp (70) :  MESSAGE: Wed Aug 29 11:32:16 LVL_ERROR--DB error -182 ErrorMessage:'Not enough fields allocated in SQLDA' from file:'.\.\src\DMTXNSQLInvoiceLineItem.cpp' at line 7612 from function:'IDBInvoiceLineItem::DBBatchLoad'

Is anyone able to give some advice on how to troubleshoot and prevent Reckon Accounts, and the machine it's installed on, from failing to read the database?

• Verifying and rebuilding the Company File has not alleviated the issue.
• Uninstalling and deleting cache preference files, and then reinstalling Reckon Accounts on the client PC has not alleviated the issue.
• As far as we know, there are no noticeable hardware issues with the desktop PC's and data networks; all other applications & web services are operating normally.
• The client desktop PC is a new-ish device running Windows 10 Pro 64-bit using a NGFF SSD, while the host desktop PC is a 5 year old device running Windows 10 Pro 64-bit on a SATA HDD.

Also, is the database that Reckon is trying to query here a temporary copy located on the desktop PC where the invoicing is being done (ie the client) - or is it the actual master database that is hosted on the Company File and Database Server is installed (ie the host)?