Error on contribution details display, recently updated system
I think it would be worth following the lead about disabled or invalid financial types. At /civicrm/admin/financial/financialType?reset=1 , do any of the financial types show as disabled? If so, you could try enabling and seeing if that resolves the error.

If you have SQL access, you could check the financial type of working vs non-working contributions and see if there's a pattern. This is in table civicrm_contribution, field financial_type_id for the contribution itself and table civicrm_line_item, field financial_type_id for the contribution's line items.

The way price sets come into it: suppose a contribution page has a price set called Subscription, with price field Journal, with price field values (options) Print and Electronic. When you set up those price field values, you specify a financial type for each. This is stored in table civicrm_price_field_value, field financial_type_id. If the user selects "Print" and this has a financial type of "Publications", say, then the contribution will include a line item for that subscription, with financial type "Publications".

If the "Publications" financial type subsequently gets disabled or deleted, that could cause issues.

If you have SQL access, this query could shed some light on the message you  received during upgrade:

select pfv.id, pfv.price_field_id, pfv.financial_type_id, ft.name, ft.is_active
from civicrm_price_field_value pfv
left outer join civicrm_financial_type ft on pfv.financial_type_id = ft.id
where pfv.label = 'Contribution Amount'
and pfv.is_active = 1;

Any results where the last two columns are NULL (indicating a missing financial type) or the last column is 0 (indicating a disabled financial type) could have caused the warning you saw.

Some price sets are used behind the scenes, which could account for "Contribution Amount" not being recognised as a field that users have set up.