I think you might be right. I'll try a different mining rig rental service and see the result.
"Your pool is shown as incompatible therefore we encourage you to contact pool operator to make sure that the pool is using minimal/starting pool share difficulty which is compatible with our service (and thus compatible with today's miners), please send them link to our FAQ on this topic:
https://www.nicehash.com/index.jsp?p=faq#faqb3. Thank you!"
Look at their FAQ:
Why is my order inactive (red) with pool difficulty to low or incorrect extranonce size error?
Unfortunately we can't permit very low pool share difficulty orders, because these are making mining inefficient and are putting serious load on our servers. Keep in mind that we usually send very high hashing speed towards targeting pool, therefore low share difficulty is putting excessive load on miners, especially on today's fast ASIC miners. These low share difficulties were left in the pools due to historical reasons, but are really inefficient with today's miners.
If you are using your own custom pool, you can easily adjust higher share difficulty setting in your pool's software (usually just one setting in the software configuration).
If you are using third party pool, it usually gives you a choice to set worker difficulty under your account page. If not, we really encourage you to contact pool operator to make this choice available or even better: ask them to rise the global minimal/starting pool share difficulty. If pool is using vardiff, the initial/starting difficulty for calculating vardiff should be set at our minimal accepted share difficulty. Furthermore, our system only accepts connections to pools with correct and standard extranonce sizes. If pool uses non-standard extranonce size you will have to contact pool's operator to fix extranonce sizes to standard ones.
Keep in mind: lower difficulties were left in the pools due to historical reasons, but are really inefficient with today's miners. That said - if your orders are disconnected with "Pool difficulty too low." or invalid extranonce sizes we encourage you to contact pool operator to make sure that pool is using minimal/starting pool share difficulty which is compatible with our service (and thus compatible with today's miners).
The required minimal initial and running pool difficulty is displayed on the "Create new order" page. Each algorithm has it's own initial and running difficulty, displayed in the field called "Minimal pool difficulty". Initial pool difficulty is evaluated while order is on stand-by without miners (but still Alive) while running pool difficulty is evaluated when order starts accepting miners - hashing power is assigned to order. If pool operator can't set this for you and setting worker difficulty isn't properly evaluated by the pool, you will unfortunately be forced to use another pool that uses higher difficulty.
Keep in mind: pool share difficulty is not the same as network difficulty.
Network difficultly is set by coin's current difficulty adjusted according to the aggregated hashing power of all active miners in the coin's network (more hashing power -> higher difficulty, less hashing power -> lower difficulty). Coin's network difficulty is regulated by coin's algorithm. Lower network difficultly puts more stress on fast ASIC miners.
Pool's share difficultly is controlled by coin's pool operator - that is the target end-user pool, not our stratum server proxy pool. It can be fixed or variable - so called vardiff. Lower difficulty puts more stress on fast ASIC miners since the miner has to process excessive number of shares in a short period of time. If miner would be working only on few shares with higher difficulty the efficiency of the miner would be higher. Also, low difficulty shares effect our system since our proxy has to process excessive number of shares in a small period of time.