You provide zero information about your network configuration and details of findings, and reply to all suggestions with "no, its not that". Then at some point the inputs will cease. You are on your own. Can you give me a contact email? As we deal with public and often "sensitive&quo...

any ideas?

The proper way to deal with the PMTUD issues is not to change MTU on either side, but rather to make sure you do not drop (block) ICMP messages that should not be dropped. A rather widespread workaround is to use TCP MSS clamping on the router (which some people consider an ugly hack- and for a rea...

Did you ping with 1400-1500 byte packet size? It is quite normal that default-size ping packets get through without problem, but larger packets as used with TCP links (1500 bytes) get dropped on bad links. Yes, in some cases we realized that the MTU was not reaching 1500, then we decreased to 1280,...

Feels like a potential PMTUD issue.

We were very confident that it was something MTU related, so we reduced the MTU to 1280 on our side and on the client side as well, but showed no improvement.

Is there anything more specific about mikrotiks that may be influencing?

I have a bad problem with my web system, well, come on ... We provide customers with a web management system where we have thousands of simultaneous customers. On customers who have any Mikrotik equipment, we are experiencing issues with errors and slowdowns due to the packet losses we have been exp...