how can i test some of the server i use?
hping3 -1 -C 3 -K 3 -i u20 <target ip>
ofc that assumes you have access to hping/hping3. I plan to check my own server as soon as I can.
We recommend that you grant permission for the ICMP unreachable message type (type 3). Denying ICMP unreachable messages disables ICMP Path MTU discovery, which can halt IPsec and PPTP traffic. See RFC 1195 and RFC 1435 for details about Path MTU Discovery