The issue might occur with any connector using Data Exchange Framework (DEF) 2.1.0 or earlier when running the pipeline batches in multiple threads. For example, if the user increases the number of threads in the Iterate Pipeline Step to some value greater than 1, the pipeline batch run might fail and cause the following error:
24104 14:15:53 ERROR [Data Exchange] Collection was modified; enumeration operation may not execute
All products that depend on DEF 2.1.0 or earlier are affected by this issue.
Download and install the hotfix compatible with the affected product version:
See the readme file inside the archive for installation instructions.
If you already have other hotfixes installed or require a hotfix for another Sitecore XP / DEF version, contact Sitecore Support.