• Make sure that the bank account and journal are set up correctly.
• check the log files to see if there are any error messages that may indicate the cause of the issue.

Facing same issue since few weeks, I tried to reconnect bank but facing below error:

It looks like our third party provider is experiencing technical difficulties with your request. Please contact Odoo support mentioning your error reference so that we can forward your problem to the third party provider. The reference of your error is 18df9133ea9f3d10. Please mention it when contacting Odoo support.

Hi, I encountered the same problem. 
(also have problem to post full answer because of my karma)

As a quick fix, replace the the 't  t - call 'AccountKanbanUploaderButton'  part of the code in the kanban QWEB view 'account[.]journal[.]dashboard[.]kanban[.]inherit' with the line bellow (add  opening and closing chevron (angle brackets), i cannot post the line with this):

widget name="account_file_uploader" linkText="Import Statement"

it will be working until next accounting module update), so consider it as a quick fix! You can write your own QWEB inherit for a more permanent solution. I will write back with a proper solution when find out exact details about this case.
regards, 
IP

All were set up correctly, as I mentioned in my question before import was done without issues, but like few week ago I've noticed that it can not be done. Thus means that smth wrong with the system. Tried to use private window, in case cache was an issue, but still same problem, i.e. no reaction when pressing Import Statement.

Reg Log files, please, advise how to check it.