Hello!
I got my home Mikrotik connected to Mikrotik at my work via PPTP connection.
All routes are made how it should, and it even worked before.
From home I can ping all devices on network, even the PPTP server Mikrotik.
When I try to connect to ANY Mikrotik at my work via I just got empty winbox, and it disconnects me after couple of seconds.
When I connect with my PC directly to PPTP Mikrotik server, everything works how it should.
What could be the issue? 
Thanks
MTU issue, set up mangle rules to reduce TCP MSS.
On which device I should set this?
I tried setting on both server and client, but still no luck.
Ensure that your firewall is allowing GRE related connections. If you don’t it will not work
Enviado desde mi Redmi Note 5 mediante Tapatalk
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I put my MAX MTU on PPTP connection to 1400 and it works great now.
It was 1450 before.
/ip firewall filter add chain=input protocol=gre action=accept
GRE overhead 24 bytes → 1500 - 24 = 1476 MTU, but you should adjust mangle to change MSS
I put my MTU lower (1400) for PPTP connection and it works great!
Thank you for advice!
After change MTU and MRU for L2TP connection bandwidth test is working both ways