Thank you, after some hours of investigation I finally found what happens (but still have no solution)
Having a query basing on several tables. In one of the result fields there is an ID referring to another table (to get the "name" linked by ID). As soon as I delete the table from the wuery, everything works fine. If I add the table and create the link (ID <-> ID) and start the query, nothing happens. I am able to CTrl-Break, but there is no progressbar, no progress even if running that query for hours.
And here is the code with and without the table "Rep_Cat_Master".
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SELECT dbo_Business_Day_Dates.BD_start_dttime, db

rder_Header.profit_center_id, IIf([tender_class_id]=1,dbo_order_item.sales_amt_gross-dbo_order_item.discount_amt,"0") AS CashRev, IIf([tender_class_id]=2,dbo_order_item.sales_amt_gross-dbo_order_item.discount_amt,"0") AS CompRev, Val([CashRev])+Val([CompRev]) AS TotalRev, db

rder_Header.bus_day_id, dbo_Profit_Center_Master.profit_center_name, qry_MaxMinDatum.Von, qry_MaxMinDatum.Bis, qry_MaxMinDatum.store_name, dbo_Menu_Item_Master.rev_cat_id, dbo_Revenue_Cat_Master.rev_cat_name, qry_MaxMinDatum.Version
FROM (dbo_Menu_Item_Master INNER JOIN ((((((dbo_Tender_Master INNER JOIN db

rder_Tender ON dbo_Tender_Master.tender_id = db

rder_Tender.tender_id) INNER JOIN db

rder_Header ON db

rder_Tender.order_hdr_id = db

rder_Header.order_hdr_id) INNER JOIN dbo_Business_Day_Dates ON db

rder_Header.bus_day_id = dbo_Business_Day_Dates.bus_day_id) INNER JOIN dbo_Profit_Center_Master ON db

rder_Header.profit_center_id = dbo_Profit_Center_Master.profit_center_id) INNER JOIN qry_MaxMinDatum ON (dbo_Profit_Center_Master.store_id = qry_MaxMinDatum.Filiale) AND (dbo_Profit_Center_Master.ent_id = qry_MaxMinDatum.Enterprise)) INNER JOIN db

rder_Item ON (db

rder_Header.order_hdr_id = db

rder_Item.order_hdr_id) AND (dbo_Profit_Center_Master.profit_center_id = db

rder_Item.profit_center_id)) ON dbo_Menu_Item_Master.menu_item_id = db

rder_Item.menu_item_id) INNER JOIN dbo_Revenue_Cat_Master ON dbo_Menu_Item_Master.rev_cat_id = dbo_Revenue_Cat_Master.rev_cat_id
WHERE (((db

rder_Header.bus_day_id)>=[Forms]![Datum]![Von Datum] And (db

rder_Header.bus_day_id)<=[Forms]![Datum]![Bis Datum]) AND ((db

rder_Item.void_type_id)<>1 And (db

rder_Item.void_type_id)<>3));
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AND WITHOUT (the working one):
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SELECT dbo_Business_Day_Dates.BD_start_dttime, db

rder_Header.profit_center_id, IIf([tender_class_id]=1,dbo_order_item.sales_amt_gross-dbo_order_item.discount_amt,"0") AS CashRev, IIf([tender_class_id]=2,dbo_order_item.sales_amt_gross-dbo_order_item.discount_amt,"0") AS CompRev, Val([CashRev])+Val([CompRev]) AS TotalRev, db

rder_Header.bus_day_id, dbo_Profit_Center_Master.profit_center_name, qry_MaxMinDatum.Von, qry_MaxMinDatum.Bis, qry_MaxMinDatum.store_name, dbo_Menu_Item_Master.rev_cat_id, qry_MaxMinDatum.Version
FROM dbo_Menu_Item_Master INNER JOIN ((((((dbo_Tender_Master INNER JOIN db

rder_Tender ON dbo_Tender_Master.tender_id = db

rder_Tender.tender_id) INNER JOIN db

rder_Header ON db

rder_Tender.order_hdr_id = db

rder_Header.order_hdr_id) INNER JOIN dbo_Business_Day_Dates ON db

rder_Header.bus_day_id = dbo_Business_Day_Dates.bus_day_id) INNER JOIN dbo_Profit_Center_Master ON db

rder_Header.profit_center_id = dbo_Profit_Center_Master.profit_center_id) INNER JOIN qry_MaxMinDatum ON (dbo_Profit_Center_Master.store_id = qry_MaxMinDatum.Filiale) AND (dbo_Profit_Center_Master.ent_id = qry_MaxMinDatum.Enterprise)) INNER JOIN db

rder_Item ON (db

rder_Header.order_hdr_id = db

rder_Item.order_hdr_id) AND (dbo_Profit_Center_Master.profit_center_id = db

rder_Item.profit_center_id)) ON dbo_Menu_Item_Master.menu_item_id = db

rder_Item.menu_item_id
WHERE (((db

rder_Header.bus_day_id)>=[Forms]![Datum]![Von Datum] And (db

rder_Header.bus_day_id)<=[Forms]![Datum]![Bis Datum]) AND ((db

rder_Item.void_type_id)<>1 And (db

rder_Item.void_type_id)<>3));
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>>>>>> Bugs will appear in one part of a working program when another 'unrelated' part is modified <<<<<