Unable to send attachments in outlook exchange

Sending attachments in outlook was always working. However, since about 2-3 weeks I realized, that everytime I try to forward an email with attachment or compose a new one, it gets stuck in my email outflow with an error message. The attachments are small (way below 10MB).
Interestingly, everytime this happens, the outlook exchange account keeps on asking for the password to be re-typed (although the correct password is in the popup). Also, when such an email is “trapped” in my outflow I will not be able to send or receive any other emails. But I will have to close the application and restart. After this step I will be able to receive and send emails again in outlook, but not with attachments.
Could you please help me in solving this issue, as of course sending attachments is extremely crucial for functionality.